Here's a thing about story telling and Halloween. To start with, forget about all the sequels, and concentrate on the original. Jonny is correct in that the original's success is built around "plausability"--rather, the notion that something of the such could actually happen rather than having a fantastical element to it. Hollywood, for what ever reason, loves to veer to the fantastical side of things, failing miserably to realize that the more real one keeps something, the more people will have a chance to relate to it. As with the best horror films, Halloween derives its premise within the family. Why? Because it's those who are closest to us that can weild the most damage in a story. Night of the Living Dead, Texas Chainsaw Massacre, Psycho, Halloween, heck--Friday the 13th... They all have this element to them about the destructive elements/forces disentigrating the American Family. Why? Because the American Family represents the ideal, the slice of the pie, the dream... blah blah blah. It's Homer in October Sky turning to murder rather than shooting off rockets. Horror, in a sense, is the antithesis of the American Dream, and what Halloween does is represent the destruction of the American Family.
Secondly, there is no supernatural element in Halloween. What does exist, however, is the mythos, and it's this success of the mythos that we sometimes feel (especially at the end) that the story has a supernatural element, when in fact, it doesn't. The mythos is set at the beginning of the film with Michael killing his sister. Why? What for? What happened to him? These are all questions that set up the myth, which over time develops. The myth of the boogeyman had existed long before Michael Myers, however, the boogeyman has never been personified. So, to the residents of Haddonfield, Illinois (or Santa Barbara, where the film was shot for that matter), the mythos of Michael Myers as the boogeyman grew and was validated by the final shots in the movie when Laurie Strode asks "Was that the boogeyman?" and Dr. Loomis replies "As a matter of fact, it was the boogeyman". This would all mean nothing at this point if it weren't for the audience taking Loomis's POV out the window to see that Michael, after being shot half a dozen times, is nowhere to be found. Now we, too, believe he is. When we speak of Mythos, it's Dr. Loomis's character that the myth lives through--without him constantly warning Sheriff Brackett, there would be no myth--so in that sense, he's little more than a plot device to let the backstory in.