I agree with Tony that this kid should be expelled and fast (and would be in the school districts around here). It may be that this would disrupt his social development, but being in an environment where you are threatened in a very real and physical sense isn't good for social development either. It's true that this boy may never actually act on his threats, but even the threat of violence is harmful.

As far as anger management being the teacher’s job, I totally disagree. It's the parent’s job to raise a child and teach him or her how to behave. If a parent can't rein the child in, there isn't a place for him or her in a public school system.