What makes a person dehumanize another? (Night book)

In terms of the novel Night, the reasons the Nazis inflicted this pain on other human beings is a complex question. The ideology of fascism, Germany's history as well as Hitler's rise to power played into the psyche of the German people. The Nazi soldiers, or at least most of them, seemed to derive satisfaction out of inflicting suffering on others. Was Hitler merely exploiting a part of our human nature that was pre-disposed to this or was all of this merely brainwashing?