Again, if Zimmerman started the fight, then he had the authority to use deadly force to defend himself only if he reasonably believed his life was in imminent danger, and he had exhausted all other means of escaping that danger.
Who started the fight is an open question. On the one hand, we have Zimmerman's statement that he had stopped following Martin and Martin jumped him. On the other hand, the testimony/records of the phone call between Martin and his girlfriend suggest otherwise.
We also have the Zimmerman call to the dispatcher - while Zimmerman was under no obligation to listen to the dispatcher's advice, the fact that he did not (as well as several of his other comments on the call, such as the reference to "a**holes" who "always get away") shed some light on Zimmerman's state of mind at the time, and are of some relevance when determining who started the altercation.