The outcome is likely to hinge on the actual harm z was defending himself against and the state of mind that the beating would have elicited.
Having been in somewhat similar circumstances, I know pretty well z's state-of-mind. The surprise at being assaulted. The adrenaline rush that follows. The sudden realisation (in my case) that the two assailants were attempting to blind me. The sense during the fight of the importance of restraint to secure one's status as the victim. Wondering if anyone is going to help.
And then, finally, deciding to take action [in my case, overwhelming the assailants and removing their weapon as evidence]. I have no doubt that in z's inferior position, I'd have taken his decision. Fortunately for me, I was less successfully harmed and better able to defend myself even though unarmed. And I had a strong witness that dispatched the falsehoods of the assailants.
If you ain't been there, you are in no position to judge how it feels. Six impacts and a broken nose is brutal and absolutely earned a response. z had no other means with which to defend himself than his gun.