I've been thinking about this, and I think I never looked at that as depressing. I actually feel like it makes the world make more sense, sort of in a "hate the sin, love the sinner" sense. I think putting people in concentration camps is depressing, but the fact that the people doing these depressing things are still recognizably human - and like us - just means that there is no Ultimate Source of Evil that we can exorcise through destruction. Fire-bombing Dresden and Tokyo depresses me too - maybe even more because we've written history to justify what we (the winners) do, and there is no catharsis involved.

Because if they're just like us that also explains why some Germans helped Jews; why Leni Riefenstahl still transformed cinema; why Knut Hamsun, a Nobel-winning writer from Norway, gave his prize to Hitler (he opposed British imperialism, and saw Nazi Germany as a force that would protect "native" Norway).

The fact that they're humans with human motivations means that those motivations can be addressed and they can be deterred. There's nothing supernatural going on; just people deciding to hurt other people for various reasons. Yes, it can happen again - and I would argue similar things have been done and are still being done - but it can also be understood, spotted, prevented, worked through.