There's no shortage in history of episodes where people have been irrational, selfish, bigoted.
Stalin and Pol Pot were just people, slave-owners were just people, inquisitors were just people, witch-burning villagers were people...
It doesn't take that much imagination to put oneself in their shoes... and see that it's largely greed, small-mindedness, or stupidity that explain what they did.
Greed, small-mindedness, and stupidity are as much a part of human nature, as generosity, open-mindedness, and cleverness. Regardless, any society in which I'd want to live promotes the latter.
There may be things that are not understandable, but, when we give it our best shot we can say we tried.
In many cases, I imagine the SS officer may have had a gun pointed to his head himself. That certainly was the case in Cambodia. If a soldier didn't kill someone who he was ordered to kill, he would be shot himself.
If we can keep tensions low and communicate better I believe we can at least forestall further world wars.
But it was the combination of the difficulty of saying no to authority and probably peer pressure , together with the reasonable possibility they'll be sent to the front lines (not even as a punishment , but because that's what men did back than).
If it's a choice between him and me, I choose me.
If it's a choice between those ten people and me, I probably choose me.
If it's a choice between a n+1 people and me, I guess I've already done n, what's another one?
Acknowledging that there is a difference between being sent away to war and certain death. That there's a difference between 1 more person and a hundred. I believe that far more people than we'd like to admit would just follow orders.
The quote was "one thousand and one is still only one thousand" from a poem about a concentration camp (https://en.m.wikipedia.org/wiki/Szebnie_concentration_camp)