All demons in my world were originally humans, whose souls went to hell when they died and had their humanity carved and burnt away. So as long as there are humans around to do things that earn them a ticket downstairs, there'll be demons. There was a way to shut them all in hell forever and prevent them from getting back up topside, but... I didn't end up doing it. Even if I should have. It's still in my future, technically, so maybe if I go back, I will.
I really hate unanswered questions, personally, so I was always going to look things up. It's definitely a mixed blessing at best, though, knowing things about your future. I tend to compartmentalize it and then focus on my future here as much as I can.
You too, Kellen. I'm not a Knight Mage or anything else official-- my world just has hunters. People who track down evil and get rid of it, as much as possible.