If God doesn't exist then why is it wrong to commit murder?
If only I had a penny for each time I've heard this question from a theist. But let's just think about what this means.
Let's assume God exists and told us that murder is wrong. Does this mean that it's only wrong because God said that it's wrong? Or does it mean that it's wrong anyway and God (being omniscient) knows this and has told us this?
1. Murder is wrong only because God said so
If murder is only wrong because God said so then there's really nothing objectively wrong with murder itself. God said that it's wrong and therefore it's wrong. If God said that it was right, it would be right. This is why it was right for the Jews to commit genocide when God ordered it. That's fine. I can't argue with this as a hypothesis. Except, a theist who holds this view must agree that they have nothing against murder itself; they only condemn it because that's what God said. If God told them to kill (or perhaps to fly planes into buildings) then they would gladly do it. Murder would be right because God said so. In fact, if murder is only wrong because God says it's wrong then murder isn't wrong at all. It's only forbidden at this point in time by an arbitary dictator (God).
When making this argument, I've sometimes had people reply to me that God would NEVER say that murder is right because God is a loving God. But if that's the case then (the multiple Biblical occasions of God commanding murder and even genocide aside) God isn't really the one who decides what's right or wrong in the first place. Rather, murder is wrong because allowing murder wouldn't be a loving thing to do. The actual measure of right and wrong isn't based on God's decision at all; it's based on an objective standard of what is or is not loving. God, as an omniscient and benevolent entity, simply follows the rules. But God didn't make those rules. So, if this is the case then God is not the answer to the question of "why is murder wrong?".
Another objection I've encountered is this: "God is unchageable. God said murder is wrong and therefore God will NEVER say that murder is right". I consider this to be a copout. If God is the true source of morality then God may want to change it. We simply can't know what makes murder wrong. God is the only one who knows. Who are we to disagree? God can even change the terms of His "covenant" (speaking the Biblical God now) with us. Nobody can stop Him. Nobody can say "but God, you told us that last one was the final one". God is the boss, not you.
2. Murder is wrong objectively; God is omniscient and has TOLD us the objective truth that murder is wrong
As I have pretty much addressed in the second paragraph of point 1 above, this means that God is not the answer to the question of "why is murder wrong?" Murder is wrong despite God and it would be wrong even if God didn't exist. God didn't create moral standards, He merely repeats them to us. We might as well take God out of the loop and instead go to the source of moral standards (whatever it is) directly. God fails to answer the question.
God can't be the answer to moral questions because if He were, we'd have to conclude that we are completely devoid of a moral radar and that murder isn't wrong after all. It's forbidden by a higher authority who could, at any time, turn around and tell us to slaughter each other and those who believe in Him would do exactly that. Is this an objective morality? I think not. Remember that the Nuremberg defence is not that genocide is morally right. Rather, it's "I was just obeying orders".