I remember an interview on 9/11 where you could hear bodies hitting the ground from the twin towers. The people on camera didn't even flinch. Stuck with me.
As a kid, I had next door neighbors with Nazi tattoos from the concentration camps. They warned me to be vigilant.
Normally decent individuals doing unthinkable, horrendous, and callous activities. No such thing as a decent, Christian, socialist.
In a former job, I had the opportunity to meet many Holocaust survivors. For some reason, they were surprised I knew about their tattoos and what they represented.
My Dad was in a Sherman that rolled over the fences at one of the camps. I posted some of his notes on that here a while back.