And, well, the thing about human history and nature is that a split morality is *natural* for us. Empathy within the family/tribe, sociopathic-like behavior to oursiders. Like, every tribe calls themselves "the People". What does that make outsiders? Not-people... And then there's the Milgram and Zimbardo experiments, showing how apparently normal people, socialized in modern societies to have unnaturally large "tribes", can still do atrocious things with a bit of social pressure.Another participant responds:
The sociopath doesn't care what he does to other people, or just doesn't respond to them as people. Normal people convince themselves other people aren't people, or deserve it, then do their atrocities.
Ah! Someone who truly understands basic human nature!
There is a descending scale of human empathy involved. Stronger loyalty to immediate kin, somewhat less so to clan, somewhat less than that to local social clique, and so on. Building large scale societies requires the creation of an abstract cultural structure (morality, religion, hierarchy, mythology), that gives humans some reason to act towards the success of the larger group instead of the smaller. When two abstract cultural structures compete without violent conflict, we call that peace. When they interact with violence and destruction, we call that war. An abstract cultural structure that can longer bind its members to its own survival is said to be corrupt and decadent.
Assigning members of different human groups a lesser moral status is as natural to humans as breathing. Complete extermination of a group happens less often than other kinds of conflict resolution only because it is rarely cost effective. Too much work, or destructive to your own cultural tropes, or because oppression and enslavement is more profitable than extermination.
Whatever we think of GENOCIDE!, it isn't crazy or even irrational to most people who practice it.
Hitler may have had serious emotional issues, but he was not an original thinker. All the terrible things he did were not the product of his imagination. He only collated ideas that had been floating around Germany for generations. He happened to have the imagination and political skill to weld those ideas into a popular governing philosophy, and didn't become clinically insane until he started losing the war and, along with it, his emotional stability and his grip on reality.