Demons in Frieren are just monsters that look like humans they are not human in the slightest.

They're worse than monsters actually. Monsters are basically animals: they have reasons for their actions and typically prefer to leave humans alone. Demons hunt humans for sport. They have fun tormenting and torturing us. (those are two very different things, eh? I hate when lazy writers use them interchangeably) It has nothing to do with race, or even species: Demons are from an entirely different Kingdom!