As you can tell, this behavior is indeed unusual. From a practical vantage point, set a firm limit with your son that such threatening comments are not permitted. Be sure he apologizes to the parties concerned. I think this behavior warrants an evaluation by a
mentalMental retardation
Mental status tests health professional because it is quite outside the spectrum of normal behavior. With a child so young, it's impossible to know what even he makes of the remarks, but it's best to proceed in a conservative way and seek an evaluation.