Hi clo,
Most of what you say is supposition.
The burden of proof is set very high for murder.
Methinks you are shooting for involuntary manslaughter because you have abandoned malicious intent.
But everything about this looks like self-defence.
M was pummeling Z. He had a broken nose, several welts, abrasions, a couple of nasty cuts. He was dizzy as a result. He was obviously worried about the cost of going to hospital (let's not forget the us health system) and took his chances. That suggests nothing about his fear of being killed up to and including the time he fired his shot. From underneath.
M needed to stop smashing Z's head. He continued his mixed martial arts assault for30 seconds.
Z was losing, in pain, panicked, taking hits to the head. He was in real danger of a brain injury. Z's response looks well within the law.
I think you must have a residual sense that Z is the baddie from the grossly misleading news reports, which suggested Z was hardly scratched (refuted by several photos at the scene) and that he was a racist (a sense that was doctored by ABC and/or CNN). 16 doesn't have to be sweet and innocent. This was a strong young man who seems to have lost his rag. Z finally had to make a decision.
Who in those circumstances would have done otherwise? Clo, you wouldn't. I wouldn't. M left Z no choices. It was survive or die.
And that ain't manslaughter. It's self-defence, however it started.
Re Geragos - yes, but a very good defence attorney. I concur with his judgement. This is more-or-less unwinnable for the prosecution.