I remember being in school back in the 60's and asking how this could have happened. (The Holocaust)
We watched movies and news reels showing how people were "dehumanized" to the point where killing them became easy.
My Dad had a book and it showed pictures of open graves about half the size of a football field full of very very thin dead people piled on top of each other.
The pictures were taken at one of the gas chambers when the soldiers got there. And they showed pictures of people who were still alive who looked like walking dead.