Without getting into heavy discussion... I do know of evil possession... but maybe some mental unwellness is caused by "possession" of some evil entity...

however, I believe we were all made perfect and because of the evil "one" we lost that (yep, Garden of Eden story).... and in that case all bad stuff comes from that evil ... I believe God is good and only gives and creates good.

We are all battling one thing or another but it's not all because of "possession" though ... and I believe that those who believe in a good God cannot be possessed by evil anyway.