I think this is part of the reason humans are so stupid during any sort of divisions where "sides" emerge. To be able to do commit this genocide, you need a very ugly "switch" in your head that can make your actions justifiable/right. I think this switch is the same, emotional, unthinking one that makes some people so religion about teams sports, phone OS, political alignment, etc.

Related, I think this is also the mechanism for how religion tends to stabilize societies/give them cohesion. Rather than having an eventual positive feedback loop of division, the division is placed between some type of "good" and "evil" rather than your neighbor. The "us vs them" division that switch craves is put on something more metaphysical (and sometimes a net benefit, like defining evil as behavior destructive to societies).