Like a spirit that's been tainted somehow. [ or maybe a spirit that had been twisted up by certain events, like the summerween trickster. there was usually some sort of trick to defeating them from dipper's experience, outside of just sending them off. ]

Yeah, the ghosts I've met haven't been outright evil, just - you know, the whole "unfinished business" thing. It's just sometimes tricky to figure out what they need done so their spirit can rest in peace.