Greentext

Do you feel the same way about zombies?

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!)

Couple of things here, demons aren’t an organic species in frieren, they’re crafted directly from magic itself to kill people. They’re 40k orcs, but slower to breed. They’re human shaped because of Darwinian mechanisms that allowed them to live longer in the shape of humans. Each any every one of them is a sociopath masking it with a malevolently high iq, with psychology of an inherently foreign base.

They’re allowed to exist freely, they choose war at every turn as their base. In fact, their king seeks coexistence and understanding, but does so by genociding humanity for a literal millennium in order to build a psychological basis for humankind’s behavior. Some demon leaders suggested that humanity would be completely wiped out long before their king might finally bridge the gap.

Attempts at coexistence continue to occur, but all turned out to be subterfuge, with the ends being genocide. There’s a vague suggestion that this is a time travel based Douglas Adams-esque god crafted world-computer created to quantize human psychology, but that is also monstrous. Anyway, read the comic if you want to understand.

Apparently the answer is “all is explained in the manga, how dare you ask this question”

Definitely not, the show is pretty much a 1:1 adaptation of the manga.

Wow the only thing true in this post is that you asked a question on stack exchange

Damn, people are real mad you don't like their elf waifu.

Downvoted for being right, the fact that more people aren't suspicious of an anime with a bunch of german names and aesthetics that also just happens to feature a race of non-people who are incurably evil and can never be trusted is baffling to me