Continuity mistake: In the scene where Laurie comes downstairs after taking a shower, she is at her front door singing to herself and she is looking out a window directly to the left of the door. Then there is a shot of the front of her house outside and you see her exit through the door that had the window next to it, but there is no longer a window there.
Continuity mistake: When Michael stabs Laurie in her arm the wound doesn't bleed at all, and there's no blood on the knife either. Then when she kneels down to talk to the kids, and in every scene after that, there's a streak of blood.
Continuity mistake: When Michael is at Tommy's school, there are cars parked across the street. When he gets in the car to leave, they are gone.
Visible crew/equipment: Just after Michael has picked up the knife, a stage light can be seen in a silver pot. This is as he walks into the next room. (00:04:15)
Visible crew/equipment: In the scene where Dr. Loomis makes a call to Haddonfield police in a phone booth by the train tracks, you can see a reflection of a crew member in the phone booth glass.
Continuity mistake: Laurie's hair keeps changing throughout the movie. Her hair changes from wavy to straight while she babysits the kids. It is wavy again when she is going to Lindsey's house, and straight again when she is running back to Tommy's house. It stays straight until Michael starts to strangle her,where it becomes wavy again, having been straight in the previous shot. It all ends with straight hair. Surely all of this could not happen in a matter of hours.
Revealing mistake: In the scene where Dr. Sam Loomis is calling from a pay phone warning of Michael Myers return to Haddonfield, a Southern Pacific freight train passes by blowing its horn (you can't see the train in the standard version). While SP trains are quite common in Southern California where the film was made, they don't actually travel through any part of Illinois. (00:19:15)
Visible crew/equipment: When the camera is zooming in on Laurie in school, the camera's shadow can be seen over the students it passes. (00:15:30)
Visible crew/equipment: When Laurie is trying to escape from the kitchen, you can see some kind of blue light in the top left corner making it look like it's light from the moon.
Deliberate mistake: The opening camera scene, that's supposed to be shot from Michael's POV as a child, is too high as noted that he's almost face to face with his parents at the end of the scene and then is only mid-chest up to the father when he removes the mask. The POV is also too high as he walks across the dining room and as he approaches his sister upstairs.
Visible crew/equipment: After Lynda and Bob make out on the couch, they both start to go upstairs. Lynda trips over the dolly track and Bob steps over it. (01:01:50)
Continuity mistake: After Tommy breaks the pumpkin, he is now walking on the sidewalk without the books that he was holding before.
Visible crew/equipment: At the end of the opening murder scene, when Michael's father takes Michael's mask off, you can see the shadow of the camera cast on Michael's father as it pulls away. (00:06:35)
Continuity mistake: Michael breaks the passenger front door window on the wagon when he takes car, a short time later when he's driving in town the window is not broken.
Audio problem: While Lynda is filing her nails just as Michael enters the scene wearing a sheet and glasses, the sound of her filing continues despite her pausing.
Revealing mistake: When Annie is putting her clothes in the wash the door slams shut and she walks over to try and open it, look closely and you can see her turn the deadbolt and lock it so she can't open it. (00:46:45)
Continuity mistake: Before Tommy breaks the pumpkin at the school in the background there are 2 kids, one with a hat holding something in his hands which disappears in the next shot.
Continuity mistake: After Laurie gets home and her room she leaves her bag and some books on the bed - they change position between shots.
Continuity mistake: When Bob and Lynda get to Annie's house he leaves the door of the truck open, but when Laurie is on the street the door is now closed.
Continuity mistake: Michael stabs his sister repeatedly. The amount of blood shown on the knife when his parents come home is nowhere near enough for the number of times he stabbed her. (00:06:35)
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