"Yes, but then we're left with the question of why those levels of killing did not go on in lands that did embrace worship of a Creator."
There's a problem with that. Consider the Islamic countries. They certainly worship a creator, yet are quite warlike and destructive.
I fear that the mere belief in deities does not preclude a society from being warlike or even genocidal.
Yes, but you're dodging the issue raised, which is that those with Atheism as their official religion are the worst, by an order of magnitude by far, when it comes to murder.