Powerful humans, unrestrained by conscience, create a hell on earth for other humans, for animals, for the ecological balance of the earth. History is written by the victors, and the victorious are by definition effective leaders. They may also be depraved. No doubt leaders of the Holocaust thought they were serving a great cause. They were willing to die for their cause, and many of them did.
Humans have a fearsome ability to rationalize.
They think they are the cause. What's good for me is what's good for the nation. And its all too easy for people to think that what's good for me and the nation is so good, so necessary, that it doesn't matter if who gets hurt on the way to accomplish it. The only thing that limits us is how much power we can accumulate. That's why we have rotating tribal wars that go on for centuries, with each group seeking revenge for the cruelties the other group inflicted when it last held power.