IIRC in most US states an "open-hand slap" is considered a much less serious form of assault than a punch (as to be fair, it's much less likely to cause serious injury) but yeah
... I understand his anger but you can't lose your cool to the point of physical assault.
If he'd just shouted clearly angry like he did afterwards I think he'd have gotten the point across. In fact, I think the audience thought it was a planned part of the show until he followed up with the shouting.