SPIRITS!!
Spirits are indeed souls of the dead that still have unfinished business. The only thing different is once the person dies, if they still have unfinished business they are stuck in limbo. The only way they can escape from limbo is if they catch a ride on someone who comes back to life. It sounds wierd so I'll put it this way, when someone is clinically dead, their heart stops and when their hearts starts again a sould has caught a ride back down to earth on their soul. Once on Earth a spirit can cause many bad things to be done, becuase after all when you are isolated from civilization, when there is nobodt to talk to, nobody to listen to, and nothing to do, you go insane. The spirit usually once on Earth gets jelous and cause bad things to happen. Deaths may even accure due to a spirits rage.
The only way to get rid of a spirit without witchcraft is help them take care of their unfinished buisness. Otherwise they will roam the Earth for all of eternity. Which probably wouldn't be good considering how much bad they could do. Yet still not all spirits do bad things some have good unfinished business like apologizing to a loved one for a fight they were in before they died and such. So excuse my stereotyping.