He didn't need to hit her because he was defending himself, and it was his only means to keep from being hurt. He hit her as a reaction, and to punish her for hitting him.

That said, I were him, I would still argue the self-defense angle all up and down. plus instinctively if you get punched in the face, it's hard to not hit back just off reflex. He's fucked though, because ultimately he did have a choice.