Not intending that Trayvon was responsible for the confrontation. It was Z's initiative that resulted in the confrontation. But that does not necessarily mean that Trayvon didn't beat the crap out of Z. How else to explain his wounds?

If Trayvon was administering an (well-desrved, perhaps) ass-beating which Z reasonably believed was mortal, then Z was protecting himself from murder. Doesn't absolve Z of moral culpability but may prove to absolve him of legal culpability.