I recall the recent news story of the group of young black men attacking the old Hispanic man, and him dying from I believe a single punch to the head? Just a little harmless game of "knock out" right?
The mother and the son in question both reassured us that he never intended or imagined the man might die... he just wanted to assault him, punch him for fun, and knock him out. Death was not intended.
But the problem is, when you assault people, especially strangers... you just don't know what might happen. People sometimes take an absurd beating and keep on ticking, other times someone will die from something you never imagined might kill them. A single punch, or a single slam of their head on concrete, it's very unpredictable.
If it's a stranger you don't know what sort of heart conditions or other frailties they may have which could influence this.
This is why it's such a bad idea, and such a dangerous thing to attack people in a civilized society. Because the results are so unpredictable. That's why we made it illegal.
The problem is, GZ and TM didn't have any sort of agreed upon fight. There was no understanding ahead of time between them that it was just a fight, not an attempted murder... it wasn't broken down into rounds and there were no ground rules.
GZ had no idea what to expect, but TM showed him a willingness to keep beating him for a pretty prolonged period of time. TM showed no receptiveness to his cries and pleas to stop.
All this adds up to it being completely understandable, morally permissible, and legal for him to have used his firearm in self-defense. And this is before we've even entered the realm of TM possibly going for the gun, which if it's true just makes it all the more clear.
A really good way to avoid situations like that where you can either be shot by your victim, or end up facing legal consequences you never imagined because they unexpectedly died from your attack... is to not assault people. It's practically fool proof.