I think that there are other ways besides religion to draw lines between groups of people (race, intellegence, economic status). I also think that fighting and bickering between people is the default position. They don't need an excuse.
So basically, I just think you're seeing causation where there's mearly correlation.
Religion doesn't cause the fighting, its just an easy thing to point the finger at.
Two human population groups living in the same place will inevitably mix over time unless they practice cultural separatism.
I just don't see that happening. People are always fighting.
The two religions involved in the history leading up to the holocaust provided that separatism, and they, along with the phenomenon of religion in general, must take a large part of the blame for the slaughter of millions.
I don't think you can blaim the religions. They were just a tool. You don't blame the gun for the murder.
Religions don't kill people, people kill people
The religions just make a good excuses, but the excuses aren't necessary.