Regarding the semantic issues discussed in these comments... no doubt that Nazis were people, not monsters (which I think of as mythical creatures, but no matter). However, some Nazis behaved in a way indistinguishable from monsters. If that is precisely what one means by calling Nazis "monsters" and doesn't mean the label literally, that seems like a legitimate (if slightly hyperbolic) use the word. (As pragmatists like to say "if the difference makes no difference, then there's no difference" or words to that effect.)

Thomas Pynchon wrote a novel called "V." (I liked it a lot better than "Gravity's Rainbow" and "The Crying of Lot 49.") One part was about how Germans slaughtered natives in one of their African colonies around 1900. He works to conjure up how it could feel good to slaughter people "monstrously." It's truly powerful writing. I recommend that part (as well as the whole novel) for anyone who wants to dive deeper into how pure monstrosity can become satisfying for humans.