To me, the supernatural has only two sources: God and Satan.
I think what we deem “ghosts” are mostly demons posing as human spirits, although I believe God does permit human spirits to roam.
I think that those who commit suicide are condemned to a ghostly existence until they repent or clear some unfinished buisiness.
When I went to summer camp, the teacher of arts and crafts told a story about how he had moved into a house with a spirirt within. This was a particulary sad story. Whenever he was alone, he sensed that someone was trying to get his attention and sometimes objects would move on their own. Their was one night where a strong thunderstorm approached his area. At midnight, he sensed something crawl in bed with him. He sensed fear, not from him, but from the very presence itself, as if it was scared of the storm. Eventually, he found out that the spiirit was that of a teenage girl who committed suicide in the 1800s. Apparently, she saw much of her father in him. At the moment he found this out, the atmosphere became almost heartbreaking and he heard the faint sound of a girl crying. I believe the spirit eventually passed over, though I cannot remember the details.