I agree, it's a difficult question. Although one philosophical argument would be to examine evil as a metaphysical phenomenon. I don't strictly think it's some kind of essential trait, in fact, I take the Stoic position that evil comes from epistemological errors, that it is often ignorance of something that is the cause of evil. That evil is not produced by something we have, but rather by something that's missing if you will.

Although in the case of concentration camps I'd say the real ignorance lies in the general public, and how they were unaware of how they could simply band together and tear those camps down brick by brick since they'd outnumber the guards a hundred to one.

That's the big lesson I believe. Next time around, if we are to learn anything from history, then it would probably be how necessary it is to join the partisans even if it might seem scary or drastic. Because even to this day people in authority use the Nuremburg defense. They're still "just doing their jobs." So I don't think we can place a lot of faith in state actors in our time sadly.