Your argument is not very good. You rely too much on rhetorical questions, which almost always belie an assumption or question-begging on your part. Second, you place too much value on your interlocutor already sharing certain parts of your worldview. An effective rhetoric must acknowledge that your interlocutor or audience likely is not in the same worldview as you. You must learn to go to them based on their own assumptions, biases, etc., and shift them over to yours. In other words, acknowledge their narrative, and then draw them into yours via a system of analogy, comparison, and demonstration of internal contradictions within their own narrative.

Here's a specific critique of some of your worst arguments:

First, if a sort of universal, objective truth exists (even one based on Scripture, whatever your doctrinal leanings), it’s arbitrarily defined.

No, by definition, an objective truth would be one capable of being intuited or rationally grasped. It would be the opposite of arbitrary.

We’ve all got cultural biases, etc. informing the way we practice our Christianity. Setting up a "right" versus "wrong" dichotomy is arrogant.

Not if there an objectively right way to practice.

Christianity is tainted with a violent history and is a haven for bigots who hardly represent Christ in the Western world today.

So is any religion, and even no religion at all (atheism). What's your point?

Technically, if God asked us to kill someone, we are obligated to do so.

Are we? This seems to be the paradox proposed to Abraham: if God commands you do to something evil, must you obey it, or is even God subject to considerations of good and evil? You should read Plato's Euthypro.

if he asks you to murder someone or an entire people, you are obligated to obey.

Again, this seems to beg the question that you must unequivocally obey a command from God, which is contradicted in Scripture by at least the example of Abraham.

Relativism doesn't necessarily equal an abysmal reality in which the greediest, etc. will triumph.

No, but you aren't responding to a claim that we necessarily will result in that state from moral relativism, only that relativism is a sufficient cause of doing so.

People don't just throw morality out the window (see: Sartre, Camus, ideological distinctions in atheism)

But can a relativistic morality ensure the sort of societal stability the original author is intending?