To understand this film, it is necessary to understand its monster. The thing in “Halloween” is usually referred to as Michael Myers, the name of the young boy in the opening. However, that’s not how Nick Castle is credited. He’s listed as playing “The Shape.” What is a shape? It is form without matter. A circle has the same nature, whether rendered in wood, ink, smoke, or mathematical notation. Thus, the Shape in “Halloween” is some form or reality that can materialize in many different ways, but always with the same nature.
Taken with Dr. Loomis’s pronouncements of the Shape’s inhuman nature, and especially with his final exchange with Strode —“It was the bogeyman!” she says, and he replies, “As a matter of fact, it was” — the implication is that the Shape is in fact a supernatural manifestation of evil. It isn’t Myers; he is only the material the Shape uses to give itself substance. This is why it always wears a mask, to the point that when Laurie briefly tears it off, the Shape pauses its assault to re-don the mask.
The Shape needs a disguise to give itself substance. It needs a “mask” of some sort. Even Myers himself is the Shape’s mask. This, of course, explains everything; The Shape cannot be killed because it is not a person but a supernatural entity. This is the same reason it has inhuman strength (enough to effortlessly strangle a German Shepherd with its bare hands) and some power over its environment (it seems able to lock and unlock doors from a distance).
It also explains the Shape’s eerily unnatural behavior. Not just its senseless murders, but the way it simply does strange things at times, such as when it appears in front of Linda wearing a ghost costume and then just stands there. Or when, after dispatching another victim, it pauses and thoughtfully tilts its head back and forth, as though studying its handiwork.
Likewise, there doesn’t seem to be any point to much of its behavior. For example, why the tableau with Annie’s body and Judith Myers’s gravestone? Or, for that matter, why is it targeting Laurie at all? The Shape, whatever its nature, is operating on a clearly alien mentality to anything we the audience can understand. It isn’t human.