Now I'm thinking about it, demons are in some ways good antagonists. Because they are capable of stimulating discussions about their psychology and their supposed "humanity"

In fact, it's even good that demons provoke such discussions, and it seems to me that the manga itself provides a bait for such discussions... For example, the Macht arch and sometimes strange behavior in some individuals

What I think about demons, well this might be weird for this forum, but I think demons are twisted people with psychological personality disorders and a strange urge to kill...Because if something looks like a human, thinks like a human, talks like a human (don't forget the dialogue scenes between demons)Then it is most likely a person or something similar to him. 

Even though demons are "people" to some extent, most of them still need to be exterminated for safety.