I've played in a haunted mansion adventure with 3 ghosts. The first two were perfectly content to be ghosts, although they didn't know why they became ghosts. The third was an evil spirit who attacked every intelligent living creature it came across. That one was actually a small child who couldn't accept being a ghost, lashing out at everything it could. It turned out that there was a curse on the family because of a deal with a devil, and they could only be put to rest by completing the deal. As ghosts they couldn't, so the party needed to do it for them. Killing the ghosts did nothing, as they'd return in 7 days if destroyed (very unhappy, as that time was spent in Hell).