What is the point of demons in this story?

Demons have been portrayed as being unable to coexist with humans. This is a question that the story has explored in many occasions and the conclusion is just “they can’t, it’s not worth it, it’s better to eradicate them.” What’s the moral here?

I just don’t understand what the purpose of having a completely irredeemable faction that must be killed no matter what and going in depth about exploring the question of if redemption is possible only to conclude with “Nope.” Like, in other stories where there’s an irredeemable faction, say the angels in Evangelion for example, the question of if they are able to feel empathy isn’t ever brought up. But if it was, and the conclusion by the end of the story is that they scanned their brain waves or whatever and found out that there is no humanity in them, what would be the point of that? Would that feel like a satisfying conclusion to that story thread? I just feel like it muddles the purpose of having a black and white moral situation in the first place.

Is the moral of the story with demons simply that there exist people in this world that are beyond saving, beyond redemption? If that’s supposed to be the takeaway then I don’t think this story is for me as much as I thought it was.