What reasons did the nazis probably have for dehumanization on the prisoners?

One was because they wanted Jews to be deported from Germany

The Nazis were under the impression that the prisoners had lost them the first world war. Hitler trained them to think like that so he could exterminate people like the Jews and prisoners whom he didn't like. So all in all, the Nazi's reason for dehumanization on the prisoners would most likely be to avenge themselves from the first world war.