Surely a physically violent response can never be justified?
It can't be justified unless it's self-defence, no.
I have posted that, while I'm not proud of doing it, I don't think badly of myself for it and have never done it since ending the marriage. The whole topic is complicated (to me, anyway, since I know so much about it now!) but, in summary, constant verbal assault is violence. Over time, it can feel like a threat to the target's very existence.
In this thread, OP has demonstrated this by self-harming in a way that historically symbolises loss of identity. It suggests she felt her husband aimed to annihilate her 'self'. If you look at the Wikipedia definition of verbal abuse, you'll see the abuser fears their partner's independent existence and wishes to wipe that out of them, as it were. So, while words are the weapon, the battle is about emotional survival. For the abuser, there can only be one winner.
Feeling one's existence is under threat, one instinctively employs measures to stop the attack. A slap isn't the most effective response in this case, but it is a common one. If one was slapped to be quiet in childhood, for example, it's an embedded reaction done without thinking.
I learned far more effective means to stop the abuse. All of them amount, ultimately, to neutralising the relationship - to stop giving the tiniest shit about what's going on in the attacker's head. Once you stop caring, of course, you will survive and the relationship's over bar the paperwork.
It's quite a big step to take, MrsLunch, emotionally and psychologically, but it's the only one that'll save you.