He slams doors, and hits people who get in his way, almost as though he has two personalities. He has been talking to people about how he hates a girl who has not returned his affections and wants to beat her up or kill her.
I read your warning signs for violent kids. He's detatched, makes threats, and has a preoccupation with violent weapons, videos etc. Once, he had a crush on me, and when I didn't return the feelings, he bullied me and at one point had me up against a wall threatening to strangle me to death.
When I ask him what's wrong, he goes quiet and says "Amy" and "family problems." I want to help him, but I'm not sure what I can do. I know he regrets how he loses control when he gets mad.
Indeed, he likely does have troubles at home, and he likley does need help -- but so do those whom he might harm.
How about giving a copy of this posting to your school headmaster? You can delete your name if that's more comfortable, but this situation is beyond your ability to handle on your own.