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.
Audio problem: When Laurie's crying near the end her mouth isn't moving in sync with the audio.
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