An interesting debate on Session 9 has been going on at IMDB. Session 9 (2001) is a movie set at an abandoned insane asylum, where a work crew led by Gordon (Peter Mullan) is removing asbestos. While one crew member listens to old psychiatric tape recordings of a killer girl who had multiple personalities, mysterious killings occur, which are ultimately traced to Gordon. At the very end of the movie, Simon, one of the multiple personalities, who had encouraged the killer girl to kill and who revealed himself on tape in psychiatric Session 9, is asked where he lives, and he answers, "I live in the weak and the wounded, Doc."
The debate on IMDB concerns the nature of Simon's existence. Some people on the discussion board think Session 9 is a demonic possession movie, in which a demonic presence named Simon who haunts the asylum possesses Gordon and makes him kill people, whereas other people think Gordon is just psychopathic and kills people for natural reasons. In the latter view, Simon was just a personality of the killer girl, with no existence outside the tape recording, and what he said about the weak and the wounded was just metaphorical. On the demonic side, one discussant makes a case for why "Simon was actually an evil spirit or entity." On the psychopathic side, another discussant writes, "To me this is clearly a movie about a man who has a mental illness."
My own view is that the film leaves the issue ambiguous, which is why different people come away with different answers. Despite the ambiguity, I do think the film leans more toward the idea of Simon as an actual demon, although the nature of Simon's comment--that he lives in the weak and the wounded--leaves it easy to see that he depends for his successes on human mental fragility. The movie need not be either/or--it may be about both an evil spirit and a man with a mental illness.
The significance of this issue for horror film history is that the presence of the supernatural is not always clear. Even a movie like Psycho, which apparently ties everything up naturalistically when the psychiatrist explains how Norman Bates came to murder various people, has a hint of the supernatural. At the end, Norman is shown locked up, and superimposed fleetingly on his face is the skull of his mother--as if she were really possessing him from beyond the grave.
Most horror movies are crystal clear about whether their deformed destructive beings are natural or supernatural. Jaws has a shark--period. Dracula has an evil being who lives for centuries and shuns crucifixes. But every once in a while the ambiguity can be great enough that audience members do not agree on what they have just seen. That is the case with Session 9.
Deformed and Destructive Beings: The Purpose of Horror Films