>Realistically, most people don't construct their life stories with themselves as the villains. Everyone is the hero of their own story. The Enemy's story, as seen by the Enemy, is not going to make the Enemy look bad. If you try to construe motivations that would make the Enemy look bad, you'll end up flat wrong about what actually goes on in the Enemy's mind.
However something like 4% of men are sociopaths, so this isn't always true. But as far as I know, Hitler was not a sociopath, nor were all of the countless other people involved.
And, of course, it's not like his actions were that different than things like the slavery of African Americans, the colonization and enslavement of 2/3 of the world by the European colonial powers (with tens of millions of dead, and heinous acts of torture and mass murder), etc.