I totally agree with those who said that he probably didn't know what he was saying. He may have heard it from TV (and it's ok if he walked on you in the room, you are entitled to some grown up TV, I don't think you did anything wrong), and for him it's a word that was used when the person was angry to let anger out and he copied that. To him it's just a word to express anger, he doesn't know how bad it is. I'm really glad the teacher just informed you about itand didn't seem like making a big deal out of it because I don't think it is. Just talk and talk and talk to your son tell him that some words are ok some are not. Give him a list of words that he can use to express his anger and talk to him about killing and death and how it doesn't teach the other person a lesson or make him stop what he is doing (which is probably all he wanted from using that word) but it has serious impact.
You may be surprised at the things that will coming out of his mouth that he doesn't know what they are (kids are like sponges there are tons of things stored in his brain already). My daughter (5 years old) says all kind of things, so far funny things, but when we ask her what does that mean, she gets confused, cause she realizes she doesn't really know what that means.
A Mediterranean 3b lady
In love with my curls, love love love
CG since Nov/07

Co-Wash: VO5, Tresemme naturals, Fructis naturals
Cond: Nature's Gate conditioners
Stylers: Lotions made at home with coconut oil, honey, AVG, vegetable glycerin, OO, and EO.
Gel: LA Looks sports gel for high humidity days.

Hair goal: Waist length healthy curls