Yes, totally. Exactly how you said it. "...he wanted a nerf war, he got a nerf war." You can't just challenge somebody, then take the most powerful weapon available, and hide behind an air conditioner, crying while you beg for mercy.
My brother would sneak in my room and shoot me. Sometimes when i visit him and dad i'll walk in and get shot in the nose or mouth... he has a Hayato sense.
That is most unpleasant and not even vaguely amusing. It isn't good to scare small children for ANY reason. This little fella is just pure traumatised, kind of psychopathic on some levels.
Whoever the 'adult' is I sincerely hope that they aren't a parent.