Did you read the top reply? It doesn't even mention the manga and explains exactly, what I would have thought was the answer to your question. Maybe the manga goes more into detail (later?), but the main parts are there:

1. Demons are an inherently evil species.
2. They are intelligent and I don't think their intelligence mimics humans, they just sometimes mimic humans to survive (like talking about their mother, which they don't have)
3. They always kill eventually and it was their goal from the get-go.

Do I find this trope boring? A bit.
Does it fit the show? I think so. Is it wrong to have such a species in a show? No.

It's fantasy and most races can work together. Having an irredeemably evil race as enemy just makes it easier to ignore ethical questions in fights (for me and a lot of other people).
Because if the show tells us, that they are indeed always and irredeemably evil, then we can ignore all those questions and just enjoy the fight. (Jajaja, that's what the Nazis ~~did~~ still do, I know!)