We had a similar incident a few months ago with my four-year-old at preschool. The preschool teacher said he got into an argument with one of his friends on the playground and said he was going to shoot him in the head. We had a serious talk with him and he's never said anything like that since. I think Liam will be the same way--he heard it from at least three sources (teacher, behavior specialist, parents) that those words are not okay and seemed very upset and remorseful.
I think it's important to keep this in perspective, too. He is not a violent or troubled kid, and it's been a one-time thing and probably won't happen again. I don't think little kids really grasp the idea of death and don't fully know what they're saying when they make those statements. Now he knows that that's very serious and knows it's not acceptable.