I agree, it is puzzling.
The main point to his book (that I got out of it) was that people have the capacity, if put in various situations, to degenerate completely.
I recall that one of the most powerful and depressing parts of that book was him wishing for his father, who was becoming a burden and attracting unwanted attention, to die, so he could live.
That is what can happen to us in a bad spot.
The worst part for me was reading his description of seeing his mother & little blonde six year old sister walking away hand in hand, presumably straight to the gas chambers. My (blonde)granddaughter was the same age when I read that.