I haven't watched the show, but couldn't the Demons evolve into not killing people and it could be what the story is trying to resolve in the end. I mean they could, it all depends on the author and what he is willing/able to write and share with us. Since the entire story is made up.

I could write a vampire story, for example, where the conclusion is that even though vampires evolved to survive on human blood and need it to survive, they resolve it by just not hunting humans, because the humans are like: "Okay we can just donate some blood to y'all? We know you need it to survive so we'll just give you some." But not all vampires are willing to let go of the hunt and not all humans are okay with their blood being donated to vampires and then it turns into another plot point where you have communist-vampire alliance vs fascist-vampire alliance and that's another story and then let's say the communists win in my story and we have total defeat of fascism, now this is the most annoying one to write because I don't know what would happen under actually existing communism, never been done before. What would be the new adversaries?

And so on.