I have heard this same reasoning applied to gang-bangers whose initiation rituals required murdering a random innocent. "Monstrous yes, but these boys are not monsters."
Frankly, I have never understood it. If monstrous acts are not what defines a monster, then what does? Surely not appearance. We are defined by what we do, not by what we say we will do, or how we are able to appear. The Nazis were monsters. Their evil is, if anything, magnified by the humdrum nonchalance and efficiency with which they often set about doing it.
The problem is most people live in a caricature of themselves.