The SYG defense requires proportionality of force.

It did not apply to Z because, pinned to the ground for with his head being thumped on the pavement for nearly a minute, he "reasonably believed" he was in danger of death or great bodily harm and was physically unable to retreat.

That would be true *whether or not* M found the gun (the DNA samples were inconclusive) and/or told Z, "You're gonna die tonight, m--f--" (for which we have only Z's word).