But, something is to be said for just flat out human craziness.
What is scarier, a human who hunts other humans and wears their skin for clothing or a human who was possessed by something to do the same? Yeah, thats a personal preference. Human depravity vs the horror of doing things you have no control over WHILE being sane and hopefully watching like a passenger in the backseat of a car.
Well, are the two mutually exclusive?
- Meatbag observer - Saying that when you're possessed, you're not just out, but you're still there. Watching. Hearing. Maybe not 100% clearly, maybe not aware you're possessed and maybe not 100% of the time. Maybe like a dream where you're drifting in/out of awareness... maybe even regaining control for periods of time.
Okay, for some reason you get unpossessed. Demon leaves, ghost is busted, etc. Do you just shudder over the horrors? Cry over the foul deeds? Or is this just the spiritual awakening you needed to learn your true calling in life? Will this just drive you totally and naturally insane? Maybe insane as in becoming a recluse and fearful or maybe insane like continuing on in the good faith your personal angel showed you that you should live life? - Voice in the Head - A few movies cover this well, we could call this the Nicholson effect or the Amnityville effect
A human is just taunted, prodded, poked, nudged and shown horrors until they're spun around and grab firmly onto reality, only thats over there and they're gripping some demon/ghosts/mages version of reality like it's the only thing that will make anything make sense.
Now in this case, there just isn't a spell to make things right. The supernatural and magic caused this indirectly, but the guy isn't possessed... or magically hallucinating... or under the effect of a spell. The guy is just insane. Maybe therapy, but maybe that'll just give you an insane therapist?
In the end, something influenced this guy to a means, and they've sure influenced this guy right down to the core of his sanity. This doesn't mean they got the exact result they were looking for or that this guy is the perfect pawn for their plans. This crazy guy could be even ounce as crazy as you can be and still reject and oppose his puppeteer.
Imagine Jack Nicholson from the Shining as a Ghostbuster! Given time, there could be other hunters who are just exposed enough to go slightly insane or full blown batguano insane. - Possession, yes but... I know in the book possession is defined as something inhabiting and/or controlling someone. What if they were just inhabiting the person? They were inside, but not in control. The ghost/demon was in the back seat? The cultist was stuck in this body without any control?
I saw one of the best concepts of this on the TV show Angel. The whole episode was about family with an evil child. Well our heroes learn that he's possessed and has been so for a really long time. They get the demon out and it's just thankful to be out. He had no control over the child and was in fact held prisoner and made to watch the child do things more evil than it wanted to do/see (in Angel not all demons are automatically the blackest of evil... there is also good, neutral, evil light, etc). Basically this demon would rather be destroyed than live inside of that kid.
Now that could go other ways. Evil demon in an Eviller human... Evil in a good soul - the demon is just powerless because of the purity of this person. Good(ish) entity trapped in an evil person?
I like twists, but I guess you couldn't just keep on throwing them out left and right and having them be twists. Eventually the party would just consider them other options to be considered when dealing with a possessed or just crazy human.