Not all possession is evil. Sometimes you have a person of intense conscience, and they become obsessed with some spiritual principle, or some story that deeply moves them. They will attract the attention of an entity that seeks to promote that. Sometimes someone can splinter off a part of themselves so much that it manifests as an independent psychic presence. It has the imprinted qualities that the originator was repressing, so often this is what people interpret as a demon. There are a variety of possible possessing forces. It comes about from a combination of vulnerability and the will to influence the world through you. Possession is both not scary and also not as weak as the movies show. There is a reality behind it, and the other stuff is mess.
“Evil, and evil spirits, devils and devil possession, are the outgrowth of man’s inadequate consciousness of God. We must avoid thinking of evil as a thing in itself-a force that works against man or, against God, if you will.” Eric Butterworth (Minister and author)