Do not underestimate people...
Ok, I, too, am a bit of an aethist, but there is one little detail about the human race you're forgeting.
Even if we didn't have religion, people would just find a different reason to kill each other.
Yes, religion is a stupid thing, and it inspires nothing but hate for others, I'll give you that. But even if the world didn't have religion, it would still be a pretty shitty place filled with hate.
It would just be a different kind of hate, that's all.