The thing with KDH is that it also establishes that there are humans that become demons and demons that are born demons.
The story is kind of all over the place on that and the fact that we have mixed children contradicts itself.
Human demons basically chose to be evil and then do nothing to combat that.
We really need more back story on Gwi-Ma and all this. Demon demons might just be literally evil. Being a manifestation of negative forces.
There also is the part where (I assume the human demons, not sure if the rest of the Saja Boys are human) they can just... not go back to "hell" and live on the surface. They choose not to do this.
I see the demons more as real world conservatives that only think they are oppressed while the go about harming people. The "good ones" are the ones that break from their rhetoric and see how they were hurting people. Basically no conservative is good, no conservative deserves to live as they have chosen the life of killing others. You can argue that they were lied to, but they keep believing in the lie instead of going "Hey!".
(This is also similar to that anime about Frieren where demons are always shown to be evil.)
The main problem with these analogues is that the "always evil" supernatural people are in fact hurting people. Vampires, Demons, Eldrich Horrors can easily overpower humans and do feed on them. It's only logical to treat them as dangerous.
