Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad."

(CNN) The death sentence for Boston Marathon bomber Dzhokhar Tsarnaev may provide some kind of justice for the families of the four people that he and his older brother Tamerlan murdered , as well as the 16 victims who lost limbs in the bombings. But the decision gets us no closer to the big question: Why do this? Why set off a bomb that kills Martin Richard , an angelic 8-year-old boy? Or Lingzi Lu , 23, a graduate student in mathematics from China?

According to Tsarnaev's own writing as the police closed in on his final hiding place, a boat dry-docked in a backyard in the Boston suburb of Watertown, it was about U.S. foreign policy in the Muslim world.

Losing blood rapidly as he lay wounded from multiple police bullets, Dzhokhar wrote on the inside of the boat with a pen: "The U.S. government is killing our innocent civilians, but most of you already know that. As a M[uslim] I can't stand to see such evil go unpunished. We Muslims are one body, you hurt one you hurt us all...Know you are fighting men who look into the barrel of your gun and see heaven. Now how can you compete with that? We are promised victory and we will surely get it."

This was an efficient rendering of Osama bin Laden's essential message that Islam is under attack by the United States and true Muslim believers who "love death" must take revenge.

So part of the "why" for Tsarnaev was the ideology of "Binladenism" that has been widely disseminated around the globe since the 9/11 attacks.

What's puzzling is that Tsarnaev was hardly an observant Muslim. As a sophomore at UMass-Dartmouth he was an easygoing, skateboarder dude, and had a reputation for partying. Twitter and Facebook accounts document Dzhokhar hanging out at a wide range of parties.

He was also a prolific tweeter. His tweets generally had almost nothing to say about the practice of Islam or American foreign policy in the Muslim world, which are the preoccupations of many militant Islamists.

His tweets generally had almost nothing to say about the practice of Islam or American foreign policy in the Muslim world... Peter Bergen

Instead, they were the typical musings of an indifferent 19-year-old American college student: Homework that was late, sleeping in, sex, girls, marijuana and alcohol.

A not untypical tweet from Dzhokhar six months before the Boston bombing said: "This night deserves Hennessy a bad bitch and an o [ounce] of weed the holy trinity."

Hennessy, "bitches" and weed are not, of course, the preoccupations of the typical Islamist zealot. In fact, it's hard to find any evidence that Dzhokhar believed in much of anything.

Coming under Tamerlan's influence

His involvement in the world of anti-American jihadism seems to have been precipitated largely by his older brother Tamerlan's deepening Islamist beliefs, although the argument he was overly influenced by his brother clearly did not stop the jury finding him guilty of the murders he was charged with and sentencing him to death.

Still, Tamerlan was the star of the tight-knit Tsarnaev family. He was the big and strong son. His mother adored him and his father, a onetime boxer, encouraged his interest in the sport. But for someone who dreamed that one day he would become a world-class boxer or musician, who drove around town in a white Mercedes (a perk from his father Aznor's car repair business) and dressed in the style of a small-time Russian pimp, favoring skin-tight leather pants, pointy shoes and billowing white shirts open to the navel, Tamerlan's actual life was somewhat less glamorous -- he dropped out of community college and then worked as a pizza delivery guy.

And, as he began to explore the Islam native to his family's homeland in the former Soviet Union, Tamerlan became increasingly disenchanted with life in his adopted country.

Tamerlan was the star of the tight-knit Tsarnaev family. He was the big and strong son. Peter Bergen

In a photo essay about Tamerlan's boxing career that was published in 2009 on a site that helped photographers promote their work, Tamerlan volunteered that he felt alienated from his adopted country, saying, "I don't have a single American friend. I don't understand them." Tamerlan also said he was "very religious" and that he had quit drinking because "God said no alcohol." In the photo essay, entitled "Will Box for Passport," Tamerlan explained that he wanted to get on the U.S. Olympic boxing team and also become an American citizen.

In 2010, Tamerlan won the Rocky Marciano Trophy, which made him the Golden Gloves heavyweight champion of New England. He was now one of the top amateur boxers in the States, but Tamerlan was blocked from advancing to the national championship of the Golden Gloves because of a recent rule change that prohibited non-U.S. citizens from competing. And Tamerlan's penchant for violence outside of the boxing ring was going to make it hard for him to obtain an American passport.

Photos: Boston bombing: The dead and injured Photos: Boston bombing: The dead and injured Martin Richard, 8, was in the second grade and loved the Red Sox. He was the middle of three children and is best known for a school project in which he made a poster with a peace sign and the words "No more hurting people." He was less than 4 feet from the second bomb. He bled to death as his mother leaned over him, begging him to live. Hide Caption 1 of 20 Photos: Boston bombing: The dead and injured Lingzi Lu, 23, was a graduate student from China, studying statistics. A gifted musician and public speaker, she was enjoying her time in the United States. She decided to watch the Boston Marathon as a study break. The second bomb sliced her leg open from hip to toe, and she bled to death. Her aunt called her "a beautiful nerd." Hide Caption 2 of 20 Photos: Boston bombing: The dead and injured Krystle Campbell, 29, was remembered by her father as his "Princess," but she loved sports and the outdoors and "wasn't a girly girl," he said. She put on big family parties and was close to her brother. She was working as a restaurant manager. She was burned and cut by hot shrapnel from the first bomb and bled to death. Hide Caption 3 of 20 Photos: Boston bombing: The dead and injured Sean Collier, 26, grew up in a big "Brady Bunch"-blended family and always wanted to be a police officer. He viewed the world from a moral stance, and felt a strong sense of right and wrong. He loved to race cars with his brother and go on family vacations. He was shot to death in his patrol car on the MIT campus because the Tsarnaev brothers wanted his gun to use in their escape. Hide Caption 4 of 20 Photos: Boston bombing: The dead and injured Celeste Corcoran, left, and her daughter, Sydney Corcoran, recovered in the same hospital room. Doctors were able to save Sydney's leg, but Celeste lost both of hers. Sydney recalled how she felt the life drain from her body and believed she was dying. Celeste felt great frustration when she couldn't rush to her daughter's side. "I never forget I'm a double amputee," Celeste said. "There's always a level of discomfort." Hide Caption 5 of 20 Photos: Boston bombing: The dead and injured Adrianne Haslet-Davis is a ballroom dancer. She remembers walking through the crowd on Boylston Street, holding hands with her husband, Adam, and feeling happy and in love. Then the bombs went off. Her husband, who is in the military, told her they'd been in a terrorist attack. But the second bomb went off before they could leave. She knew immediately something was wrong with her foot, and could see blood everywhere. She couldn't hear her own screams and thought she was dead. They were dragged into the Forum restaurant, and a first responder recalled hearing her husband apologize to her over and over for bringing her to the event. She testified that he recently checked himself into a Veterans Affairs mental health program. Hide Caption 6 of 20 Photos: Boston bombing: The dead and injured Erika Brannock was the last bomb survivor to leave the hospital. She came to the marathon from her home in suburban Maryland to cheer her mother on and was excited about being able to get so close to the finish line. Her sister, Nicole Gross, testified that she recalled pushing Brannock through the crowd so they could get closer. "I said, 'One, two, three, go,' and as soon as I said, 'Go,' the bomb went off," Gross said. Brannock told reporters in 2013 that she saw flashes of orange and yellow light and was knocked to the pavement. She couldn't move her foot and thought she was going to die. She said she's had moments when she was angry with the bombers but "I can't waste my energy being angry. I need to save energy for getting well and for being with people who care about me and want me to get better." Hide Caption 7 of 20 Photos: Boston bombing: The dead and injured Jeff Bauman remembers looking down at his legs. "It was just pure carnage. I could see my bones and the flesh sticking out, and I just went into tunnel vision. I thought this is really messed up, this is messed up, that's all I said in my head. This is how it's going to end. This is it. I had a great life. I saw the world. I played sports growing up. I had a lot of friends ... I made peace with myself at that point." Hide Caption 8 of 20 Photos: Boston bombing: The dead and injured Heather Abbott was outside the Forum restaurant when the second bomb went off. "I was catapulted through the doors of the restaurant, which was open. I landed in a puddle of chaos and blood and glass. People were running in herds by me, through the restaurant to get to the back exit, away from where the bomb was." Her foot was badly mangled, and she decided to amputate below the knee. She wears a prosthetic with toes and a high heel. Hide Caption 9 of 20 Photos: Boston bombing: The dead and injured J.P. Norden, left, and his brother, Paul Norden, did not testify during the trial, preferring to put the bombing behind them. Their mother, Liz, is outspoken in the survivor community. Hide Caption 10 of 20 Photos: Boston bombing: The dead and injured Karen Rand McWatters lost a leg -- and one of her best friends. She and Krystle Campbell spent the day laughing and posting selfies on Facebook before heading to the finish line. She was knocked to the ground by the first blast, and heard the second before she could understand what was happening. Her foot was turned in the wrong direction, but she dragged herself toward Campbell. She couldn't see how badly hurt her friend was. "I got close to her head, and we put our faces together. She very slowly said her legs hurt, and we held hands and very shortly after her hand went limp and we never spoke again." Hide Caption 11 of 20 Photos: Boston bombing: The dead and injured Mery Daniel, a young mother from Haiti, was attending her second marathon when she was knocked to the pavement. She lost a leg, and the other leg was also damaged. She had pushed herself from ESL classes all the way to medical school, and instead found herself relearning how to walk with a prosthesis. She wears her "Boston Strong" T-shirt proudly. Hide Caption 12 of 20 Photos: Boston bombing: The dead and injured Marc Fucarile was at the marathon with friends, and was struck by hot shrapnel from the second bomb. His pants caught fire, and he suffered burns over 90% of his lower body. His belt buckle was so hot, it burned his hand when he tried to undo it. One leg was blown off at the scene, and he's still trying to save the other, but might not win that battle, he said. He's had more skin grafts than he can remember. Hide Caption 13 of 20 Photos: Boston bombing: The dead and injured Patrick Downes and Jessica Kensky Downes met when they were interns on Capitol Hill. She lost both legs and was pushed into court in a wheelchair. Her aide dog, Rescue, lay beside her as she testified. "I remember being happy, I remember feeling sunlight on my face. I remember feeling free." And then the bomb went off. Because she is a nurse, she focused on saving her husband. His foot and part of his leg were hanging by a thread. She remembers screaming, and not being able to hear anything. This photo was taken before she decided to amputate her second leg in January. "I wanted to paint my toenails and put my feet in the sand. I wanted all of those things, and to lose my second leg was a gut-wrenching decision." Hide Caption 14 of 20 Photos: Boston bombing: The dead and injured Rebekah Gregory was celebrating her birthday weekend at the marathon with family and friends. She remembers coming to after the blast and reaching for her 5-year-old son, Noah. She could see bones protruding from her arm. She also lost a leg below the knee. Like many bomb survivors, she was convinced she was going to die that day. She is using a blade to run again. After she testified, she wrote a note to Dzhokhar Tsarnaev on her Facebook page saying she is not afraid of him. Hide Caption 15 of 20 Photos: Boston bombing: The dead and injured Roseann Sdoia had run a 5K the day before the marathon. She heard the first bomb and decided to run for it. Then she saw two explosions of white light at her feet. She knew she'd lost a leg before she hit the ground. She saw a severed foot with a sock and remembers asking herself if she wore socks that day. "It was somebody else's foot." She thought she'd rather die than live as an amputee but then considered all the people she'd be leaving behind. So she willed herself to stay conscious and fight. Hide Caption 16 of 20 Photos: Boston bombing: The dead and injured Jane Richard, in the pink skirt, lost her leg. She holds the hand of her brother Henry as they walk down Boylston Street with their parents and others after an April 15 ceremony this year. She was standing next to her brother Martin behind a metal barricade when the second bomb went off. Her father, Bill, took one look at Martin, knew he wouldn't make it and focused his efforts on saving Jane. She sang in April at Fenway Park on opening day. Hide Caption 17 of 20 Photos: Boston bombing: The dead and injured Mary Jo White and Bill White, right, had gone with son Kevin into the city for lunch and were on their way back to "the T," as Boston's public transportation system is called, when they decided to stop by the finish line. They were just feet away from the first bomb. Bill, a veteran who earned a Purple Heart in Vietnam, lost his leg above the knee. Kevin, who serves as the family spokesman, says his parents, who are in their 70s, don't like to talk about the events of April 15, 2013. He told his local newspaper he "really has no feelings" for Dzhokhar Tsarnaev. Hide Caption 18 of 20 Photos: Boston bombing: The dead and injured Steve Woolfenden, a biomedical researcher, was pushing his son, Leo, in a stroller along Boylston Street. His wife was running the marathon, and they were making their way to the finish line when the first bomb went off. He started to turn the stroller around in the crowd, but the second blast caught them. Woolfenden's leg was severed on the scene. He could see it still in the boot next to him, the tibia protruding. His focus was on Leo and getting him help. Hide Caption 19 of 20 Photos: Boston bombing: The dead and injured Leo Woolfenden was lifted from his stroller by a first responder as the boy's father lay on the ground with a severed leg. Leo suffered a skull fracture.

Hide Caption 20 of 20

At the same time that his boxing career was blossoming, Tamerlan for the first time began attracting the attention of law enforcement. On July 28, 2009, Tamerlan hit his live-in girlfriend. "Yes I slapped her," Tamerlan told the police when they interviewed him about the incident. The case wasn't prosecuted, but it likely held up his later application for U.S. citizenship,

Around the same time, the family patriarch, Aznor, got in an altercation at a Boston restaurant and was struck in the head with a steel pole, an injury he never quite recovered from. Then Aznor's car repair business took a dive.

The family now subsisted on food stamps and welfare payments. The parents divorced and Aznor went back to Dagestan, while his wife, who had once dressed in high heels and tight skirts, adopted a veil and found a new identity in fundamentalist Islam. In 2012, she was accused of shoplifting $1,600 worth of dresses from Lord & Taylor. She fled to Russia and records suggest she remains wanted on a felony charge

Bella and Ailina, the couple's daughters, moved out of the family home in Cambridge, settling in New Jersey where Bella was subsequently arrested on a charge of distributing marijuana and she entered a pretrial intervention program

The Tsarnaev family was fracturing.

Going nowhere

In June 2010 Tamerlan married Katherine Russell , his on-and-off girlfriend. They had met by chance in a downtown nightclub. Russell converted to Islam and took a new Muslim name, Karima, and started wearing the body-enveloping burqa. The couple soon had a baby girl.

Tamerlan and Russell lived on welfare payments for a while, but those ran out. Russell started working as many as 80 hours weeks as a home health aide, while Tamerlan was unemployed.

Once a possible contender for the Olympics, Tamerlan was now jobless and dependent on his wife for any income. As he drew deeper into the world of Islam, Tamerlan quit boxing entirely and stopped wearing flashy clothes, occasionally wearing a bathrobe over his habitual uniform of sweatpants and T-shirts.

Tamerlan was now a very long way from his conception of himself as a larger-than life hero. Peter Bergen

A schlub going nowhere, Tamerlan was now a very long way from his conception of himself as a larger-than life hero.

At a Boston mosque Tamerlan stuck up an unlikely friendship with Donald Larking, an elderly invalid who had recently converted from Catholicism to Islam . Larking and Tamerlan were fellow conspiracy theorists. Tamerlan believed that 9/11 was an inside job and that the government had pulled it off.

Tamerlan's mother, Zubeidat, reportedly also believed that 9/11 was engineered by the U.S. government to create mass hatred for Muslims. Her views of the "real" causes of 9/11 were shared by both of her sons.

In addition to his fringe beliefs about 9/11, Tamerlan also started recommending to acquaintances "The Protocols of the Elders of Zion," which purports to be the secret plan by the Jews to take over the world. It was long ago debunked as a century-old Russian hoax.

The YouTube account maintained by Tamerlan took on an increasingly militant tone. He posted a well-produced video about the prophesy that an Islamic army carrying black banners would appear from Khorasan -- an ancient name for a region that encompasses Afghanistan -- to bring true Islam to the Muslim world. The prophecy is a key part of al Qaeda's self-image as the black-banner-carrying army emerging from Afghanistan that is destined to bring back the caliphate. The video that Tamerlan posted to YouTube showed al Qaeda fighters training in Afghanistan.

Like many other jihadi militants in the West, the Tsarnaevs downloaded Inspire, the webzine put out by al Qaeda in the Arabian Peninsula.

Photos: Boston Marathon bombing evidence Photos: Boston Marathon bombing evidence A jury condemned Dzhokhar Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more in the 2013 Boston Marathon bombings. See photos that were released as evidence in his trial. Hide Caption 1 of 35 Photos: Boston Marathon bombing evidence This undated photo of a young Tsarnaev with his brother, Tamerlan, was shown by the defense in the sentencing phase of the trial. Tamerlan died after being shot by police and run over by a car driven by his brother in the massive manhunt that followed the bombings. Hide Caption 2 of 35 Photos: Boston Marathon bombing evidence Katie Russell met Tamerlan Tsarnaev at a nightclub and dropped out of college to marry him. Her mother, Judith Russell, testified that Tamerlan came between Katie and her family and that Katie became isolated. She eventually converted to Islam and changed her name to Karima Tsarnaeva. She was the breadwinner. But when company came for dinner, she cooked, served the men and then retired to another room. Hide Caption 3 of 35 Photos: Boston Marathon bombing evidence This collection of photos of Dzhokhar Tsarnaev in his wrestling days was introduced by the defense. Hide Caption 4 of 35 Photos: Boston Marathon bombing evidence This photo of Tamerlan Tsarnaev, recovered from his computer, was shown during the sentencing phase. Hide Caption 5 of 35 Photos: Boston Marathon bombing evidence This image shows victims' positions in the crowd prior to the Boston Marathon bombings on April 15, 2013. Hide Caption 6 of 35 Photos: Boston Marathon bombing evidence Tsarnaev "flips the bird" in a jail cell during his first arraignment on July 10, 2013. The image was presented to jurors in the sentencing phase of his trial. Hide Caption 7 of 35 Photos: Boston Marathon bombing evidence Tsarnaev poses in front of a black standard adopted by various militant Islamist groups in this Instagram photo that was entered as evidence. Hide Caption 8 of 35 Photos: Boston Marathon bombing evidence Prosecutors say Tsarnaev was a self-radicalized jihadist who pored over militant writings, including the article "How to Build a Bomb in the Kitchen of Your Mom." It was found on his laptop and other devices, part of a full-edition download of Inspire magazine, a glossy English-language propaganda tool put out by al Qaeda in the Arabian Peninsula. Hide Caption 9 of 35 Photos: Boston Marathon bombing evidence This Russian manual on how to fire a handgun was found in the apartment where Tsarnaev's brother, Tamerlan, lived. Tamerlan Tsarnaev was killed in a shootout with police in Watertown, Massachusetts, on April 19, 2013. Hide Caption 10 of 35 Photos: Boston Marathon bombing evidence This copy of The Sovereign, which calls itself the "newspaper of the resistance," was also found in Tamerlan Tsarnaev's apartment. Hide Caption 11 of 35 Photos: Boston Marathon bombing evidence Pictured here is a box of bullets found on a street after the shootout in Watertown. The brothers' fingerprints were on the box, prosecutors said. Hide Caption 12 of 35 Photos: Boston Marathon bombing evidence A pressure cooker was embedded in the side of a resident's Honda during the Watertown shootout. Hide Caption 13 of 35 Photos: Boston Marathon bombing evidence Photos of the Watertown shootout were entered into evidence. Neighbors came to their windows and then retreated. One grabbed his infant son and headed toward the back of his house with his wife. Another grabbed a camera and took photographs from an upstairs window. Hide Caption 14 of 35 Photos: Boston Marathon bombing evidence The Tsarnaevs had carjacked a Mercedes SUV in Watertown before the shootout. The vehicle was covered in bulletholes, and the rear window was shattered. Hide Caption 15 of 35 Photos: Boston Marathon bombing evidence This unexploded pipe bomb was found at the scene of the shootout between police and the Tsarnaev brothers in Watertown. Hide Caption 16 of 35 Photos: Boston Marathon bombing evidence Prosecutors said these boards were attached to the boat where police found Dzhokhar Tsarnaev hiding. A carved message reads, "Stop killing our innocent people and we will stop." Hide Caption 17 of 35 Photos: Boston Marathon bombing evidence Smashed phones and an ATM card owned by carjacking victim Dun Meng were in the yard where Dzhokhar Tsarnaev was found. Hide Caption 18 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this surveillance image shows Tsarnaev visiting an ATM hours before a police chase and chaotic shootout in which more than 200 rounds were fired. Hide Caption 19 of 35 Photos: Boston Marathon bombing evidence Another view of Tsarnaev's visit to the ATM. Hide Caption 20 of 35 Photos: Boston Marathon bombing evidence Exhibits related to the shooting death of MIT Officer Sean Collier were introduced to the jury on Wednesday, March 11. This image from the crime scene appears to show a bloody gun. Hide Caption 21 of 35 Photos: Boston Marathon bombing evidence This burned tank top and yellow hoodie belonged to bombing survivor Jessica Kensky. Hide Caption 22 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this Fox Racing logo was from one of the backpacks containing a bomb. Hide Caption 23 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this still image from surveillance video shows Tsarnaev in the UMass Dartmouth gym the day after the bombings. Hide Caption 24 of 35 Photos: Boston Marathon bombing evidence Prosecutors showed the jury photos of what they say are Tsarnaev's writings inside the boat he was captured in. Hide Caption 25 of 35 Photos: Boston Marathon bombing evidence This image is from a surveillance camera outside the Forum restaurant in Boston's Copley Square just after the bombing. Hide Caption 26 of 35 Photos: Boston Marathon bombing evidence Prosecutors presented two Twitter accounts linked to Tsarnaev that, they said, showed targeting the marathon had been on his mind for at least a year. One account, @J_tsar, contained 1,100 tweets and was the more mainstream of the two. On the day of the 2012 Boston Marathon, a tweet from the account read, "They will spend their money & they will regret it & they will be defeated." Hide Caption 27 of 35 Photos: Boston Marathon bombing evidence Prosecutors said the second Twitter account is evidence that Tsarnaev led a double life. By day, he was a slacker college sophomore. By night, he was a wannabe jihadist, posting on the account @Al_firdausiA. In one tweet, he urged people to listen to radical cleric Anwar al-Awlaki's lectures. "You will gain an unbelievable amount of knowledge," he said in March 2013, just weeks before the bombings. Prosecutors also allege in an indictment that Tsarnaev downloaded al-Awlaki's writings, calling him a "well-known al Qaeda propagandist." Al-Awlaki had been killed in a U.S. drone strike in 2011. Hide Caption 28 of 35 Photos: Boston Marathon bombing evidence Hide Caption 29 of 35 Photos: Boston Marathon bombing evidence A Boston Marathon bombing victim is tended to in the street. Hide Caption 30 of 35 Photos: Boston Marathon bombing evidence Victims at the finish line just after the bombing. Hide Caption 31 of 35 Photos: Boston Marathon bombing evidence Eight-year-old Martin Richard, the youngest victim , can be seen standing on the rail in the front row. Hide Caption 32 of 35 Photos: Boston Marathon bombing evidence A closer view of 8-year-old Martin Richard in the crowd before the bombing. Hide Caption 33 of 35 Photos: Boston Marathon bombing evidence Boston police tend to a wounded child. CNN has chosen not to show the young victim's face. Hide Caption 34 of 35 Photos: Boston Marathon bombing evidence Mayhem along Boylston Street. Hide Caption 35 of 35

The first edition of Inspire, which appeared in the summer of 2010, featured an article about making bombs from pressure cookers and gunpowder from fireworks, a bomb-making recipe that appears to have influenced the Tsarnaevs.

Dzhokhar Tsarnaev couldn't resist telegraphing his forthcoming role in the Boston bombings. For the first time in his young life he was going to be someone other than the slacker barely-getting-through college student and small-time drug dealer that he really was. In the weeks before the attacks, Dzhokhar tweeted to his followers: "If you have the knowledge and the inspiration all that's left is to take action."

The action Dzhokhar took at the Boston Marathon seems to have emerged from the lethal combination of the ideology of Binladenism, which has taken on a new life with the proliferation of English-language jihadi propaganda such as Inspire; his brother's disappointments in life; his deepening Islamist beliefs; and the collapse of the Tsarnaev's family life.

But this still doesn't really answer question "Why?" After all, plenty of people object to American foreign policy in the Muslim world and many people have disappointments in life, and many families fall apart, but few turn to violence for these reasons.

Two books have been written about the Tsarnaevs; hundreds of news articles have been written about them, including an excellent series by the Boston Globe ; the trial of Dzhokhar Tsarnaev produced many thousands of pages of testimony, yet we are still no closer to having a totally convincing explanation of why the Tsarnaev brothers from Cambridge murdered their fellow citizens.

Perhaps this says something about the fundamental nature of evil acts; you can point to why a convicted mass murderer might have disappointments in life or be swayed by bin Laden's ideology, but the Tsarnaevs' acts are ultimately inexplicable.