My hunch is that this was the case, but a trial was demanded. Again, perhaps they lost because they couldn't win, perhaps because they didn't try. I couldn't tell the difference.
But you wanted made up evidence. Sure. I'll give it a go.
You the prosecutor need to show not that Z was not in fear for his life, that's not what you're after. You need to show that a reasonable person would not be in fear for his life and therefore would be reasonable in the use of deadly force. You also have to show that Z either had no opportunity to retreat or, if given the opportunity failed to use it.
If Z were on top and forensics said that M had an impact to the skull that would have produced unconsciousness. (1) Z would have opportunity to escape and (2) no reasonable person would be in fear for his life from an unconscious M.
If Good had testified that he had seen clearly that the guy in the darker sweatshirt was on the bottom and he had heard him say, "Mister, I give up, stop hitting me." No reasonable fear.
If it could be shown that M suffered from a cardiac ailment that rendered him weak and unable to run. Or had been suffering from an attack of asthma. Again, no reasonable person would be in fear.
If the ballistics said that the bullet had gone in level and from a distance of 10 feet. No unarmed kid is going to be a threat at a danger of 10 feet--that's not reasonable. Moreover, Z would have more than ample opportunity to retreat.
See. Not hard. Making up evidence is easy, and in a lot of cases the self defense claim isn't used because it's a waste of everybody's time. This was a crappy case. the witnesses were crappy. The evidence was crappy. The defendent was crappy.