It seems to me that demonizing and dehumanizing someone like Hitler is a coping mechanism that allows us, as humans, to go on. How could we go on otherwise, if a person could do what he could? How can we go on if we didn’t stop him?
The danger is that by demonizing someone like Hitler, we help blind ourselves to people like him that come after. We deprive ourselves of the understanding we need to recognize that growing evil in someone.