Or, he said it in a way that made the threat non-credible. Think back to the days when kids of all ages would say things like, "I'll pop a cap in ya!"
There's a huge difference between something said in a fit of anger (generally only credible in the moment) and something said during a period of calm, "I'm going to kill all these people." If it wasn't a credible threat (and it doesn't make sense that it would be), then it also doesn't make sense to boot him over something he said while mad as hell.
Hopefully, this puts him in the counseling, classes, etc. that he needs to get his **** straight - forever.