Hi Cactus Flower,
Z was in pursuit of T, (even though he was told not to) T continued to try to get away from him.
At the end, T turns & Z is back & confronts him.
Z didn't say that he was part of the neighborhood watch, maybe he did tell him he had a gun.
T now feels threatened & in fear of his life & so the fighting starts.
T is the person in the self defense situation, he doesn't have a gun, only his fists. He isn't alive to present his side.
Maybe that's why Z was calm after the fact, he always knew he had the ace in the hole...
IMO, if Z felt threatened, or even the fact he just killed a person, I would expect he would NOT be calm.
People testified he was in fact calm.
IMO, that has Z appear more like a cold blooded killer.