In my opinion, there was a lot more at play than just angry, shouted words. Luna was playing mind games, partly to do with her resentment of his neglect of her away from what was going on in the motel room itself and the mc's mind was already very fragile, his temperament clearly fractious and his self-restraint highly volatile from the suicide of his student for which he was still in the middle of therapy. When Luna saw that he was losing his self-control, she goaded him over and over to hit her, he lost control, he snapped and lashed out once, then upon coming to his senses was immediately sincerely remorseful.
As far as I am aware and I always stand to be corrected, aggravated assault, whether controlled or not, is usually intended to physically injure and incapacitate. This, in my opinion, was not the case or the objective in this instance and I think Betisis made a noticeable effort to convey this in the resulting scene.
For all these reasons, to me, the emotions, the bitterness (not towards each other) and the frustrations on both sides ran a whole lot deeper than just "neener neener". I am not in any way attempting to excuse the punch, I am trying to indicate what I thought were the forces in play that contributed to and ultimately caused the punch to be delivered.