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 to exist 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.