The Dark Ones by Bryan Smith was one of the more disturbing supernatural horror novels I've read. It's about a group of teens who are essentially outsiders. They are the titular Dark Ones. One night they mess around in an old, abandoned house and accidentally unleash a demon. The demon possesses one of them and starts to turn the townspeople into sex-crazed killing machines.
Besides all this, there's the generally crappy lives of some of these kids. One of them has a mother who is quite literally insane. Another was a victim of sexual abuse as a child. A third get bullied by the jocks. In its way, that was even more disturbing than the demon stuff.
This is a gory and disturbing book that really isn't for the faint of heart. For the rest of us, it's a worth reading.