On Friday morning, an ordinary-looking elderly gentleman will walk to a small white X painted in the middle of a street in downtown Dallas and wonder for the millionth time what difference a single second could have made in his life, the life of a president and the destiny of a presidency.

For him it’s absolutely personal. Friday is the 50th anniversary of the assassination of John F. Kennedy. And that faded X marks the spot where the fatal bullet hit the president in the head as he rode by in his motorcade.

The elderly man is Clint Hill, the only Secret Service agent who sprinted toward John and Jackie Kennedy after the first shot was fired that day, reaching the back of the limousine just in time to see the final devastating shot.

For the last 50 years he has blamed himself, despairing that if he could have gotten there one — or even half of one — second faster, he could have taken the bullet for JFK.

“I was one of a group of agents who failed to fulfill our responsibility,” Hill, now 81, said. “He was killed on our watch, and we failed. That’s how I have always thought about it, and I always will. The 50th anniversary will be no resolution for me in that regard.”

Images of the Secret Service agent desperately hauling himself onto the back of the accelerating limousine as a panicking Jackie Kennedy scrambled out onto the trunk are indelible.

The experience tortured Hill to the point where he contemplated suicide at times over the years, he revealed to Al Jazeera in an interview this week.

“I thought about it, sure,” he said. “I sometimes thought it was not worth carrying on.”

The notion of leaving his two sons fatherless, like Kennedy’s children, combined with the influence of his Lutheran upbringing dissuaded him, he said.