But you witnessed that. I worked in mental health and I'm a medium so as far as this goes I've seen both sides of the coin, and I also have my own mental health issues. I've seen people go on full-blown psychotic episodes like you see in the movies. In one case I had to 'trap' a young woman in a public toilet, she sat in a puddle of her own urine screaming at me, and herding her into the toilet was preferable to her running into a busy main road to get away from me. I can only guess what she saw me as, but not long prior to that I'd been doing some training with her and having a laugh. What she saw was all in her own mind. There have been other cases but there is a difference between the demons in people's heads and something more real or objective similar to what you witnessed.
If you witnessed it then what was going on wasn't just in that person's head. There is a difference. I don't have an explanation for that but I've also witnessed things that can't be explained by any kind of common sense even. Something exists beyond science that's for sure.
jro made interesting observations and indeed it's not easy to explain exactly what was going on. So-called demonic possession isn't new and neither are reports of levitation. Thankfully they're comparatively rare (I think? Depends on what you compare.) and although intriguing such reports don't add anything substantial to what's already known about our survival in a different form beyond physical death.
Let's not forget, though, that this thread is titled: "Schizophrenia and Bipolar Disorder?"