A big problem with many of the nazi soldiers who participated in those monstrosities was that they didn't believe that they were actually committing them. For starters, there was constant propaganda that the Jewish people were not humans, more like civilized animals, and they didn't believe them to be humans. Another was that they considered themselves to just be doing their job. They didn't think that they were doing bad things because they got their orders from higher ups and they just did their job.
