Why are people sure he was actually possessed?
Can't there be another explanation for his behaviour? I thought it was giving massive hints that he was his own undoing, and everything external that happened to him were all mental triggers.
He's quite unstable to start with, then took all manner of hallucinogens, including acid, and went though a bunch of hideously traumatic events. He then got totally obsessed by it all, blacking out for large periods of hallucinations and self torment and torture.
But it's not necessarily a 'possession' in the religious sense of the word.
He went off his nut! - is another explanation.