It was a slow, irreversible process. Antidotes to poison are developed by graduate increasing the amount of poison. Violence is induced in a similar manner. The main purpose was the degrading of the victims in the eyes of the killer, to make his job easier. It was Primo Levy that formulated this idea.
Cruelty is very contagious.
To understand the mechanics of the Holocaust, we have to recognize the overbearing power that a Totalitarian state has over his citizens. Stalin starved, shot or froze to death about 20 millions Russians. He had plenty of willing executioners but nobody is questioning his responsibility for the carnage. Pol Pot murdered at least 1.5 millions of Cambodians. The executioners were Cambodian, but nobody blames them for the massacres.