Question: Why was Michael killing people? There was no mention of his history, or what made him the way he was.
Answer: The movie doesn't require a back story, although subsequent sequels, and the Rob Zombie remake address your questions. Then again, what makes any serial killer kill? The topic has been studied by psychologists for decades. Often serial killers lead normal lives, at least in public.
Question: Why does Leonard Nimoy have a brown piece of leather on the back of one of his hands?
Answer: On the DVD commentary, Director Phillip Kaufman explained that the leather on Nimoy's hand was the actor's idea and simply showed how kooky the character was. Fits the character well really.
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