Trivia: Michael Myers' mask is a Captain Kirk mask painted white with ruffled hair and a few other tweaks.
Visible crew/equipment: Just before Laurie stabs Michael with the knitting needle you can see that one of the window shutters is open and that the wind is ruffling the curtain. When the camera gets closer and the curtain is out of the way you can see the huge fan in the studio being used to simulate the wind outside. (01:19:50)
Continuity mistake: When Annie drops Laurie off at Tommy Doyle's house, when she walks in their front door it has multiple glass panels, but later it is a solid, plain white door. (00:34:45 - 00:39:55)
Continuity mistake: In the scene where Laurie walks across the street to check on her friends, the porch of the Doyle house has nothing on it. When Laurie runs back across the street and starts pounding on the door she picks up a flower pot off the porch to throw at the window. (01:11:20 - 01:18:30)
Visible crew/equipment: During the opening sequence, the scene is shot in first-person point-of-view from the young Michael Myers. As he walks through the house, the shadow of the camera can be seen several times. After he picks up the knife in the kitchen, the camera's shadow is visible on the door frame leading into the dining room. It can be seen again just before he turns to go upstairs. (00:04:05)
Continuity mistake: When Laurie escapes the house by smashing a window pane, the glass shards disappear instantly in the next shot. (01:17:47)
Answer: The only answer given in this film is that Myers is purely and simply evil. He's just doing it because he's compelled to, and doesn't seem to have any trace of humanity left inside of him. Future sequels attempted to give an explanation, but to varying degrees of success. But as far as this original film is concerned - he's just pure evil.
TedStixon