It does not matter, according to that law, if GZ had life threatning injuries...only what GZ believed about the danger he was in as TM broke his nose, straddled him and pounded his head into the ground.
"Deadly force is justified if "the person reasonably believes that he or she is in imminent danger of death or great bodily harm and that he or she has exhausted every reasonable means to escape such danger other than the use of force which is likely to cause death or great bodily harm to the assailant;"
Witnesses gave statements that GZ was crying for help...TM was not stopping..so all reasonable means were "exhausted." I'm reminded of the movie star (Vanessa redgrave, I think) who died after a fall skiing...it doesn't take much sometimes with head injuries amd you don"t need blood.