I mean, the only rational explanation is that the Nazis were using mind control devices on their own people to force them to be inhuman monsters. There was probably some resistance among the ranks, but that was quickly countered with the electrodes and Jewish antennae. As an American, I find it impossible to believe that an entire nation could voluntarily have chosen to have done the things that Nazi Germany did.
On the other hand, if they weren't controlled by mind control, then the only logical explanation is that Germans are monsters that enjoy doing horrible things to humans.
You might want to read the excellent book "Obedience to Authority" by Stanley Milgram, or at least the Wikipedia article that summarises it.
Also, it is worth noting that Hitler did not suddenly start the Holocaust. Rather, when he came to power in 1933, he put in several years of preparation to demonise Jews. He did this by a combination of techniques, including: introducing increasingly more severe anti-Jewish laws; changing the school curriculum to teach his theory about racial hierarchies; taking control of the German newspapers and radio stations so he could disseminate propaganda to the German population; censoring any woks of art, literature, music and science that had been created by Jewish people; and requiring all children to join Hitler Youth.