All that matters is if at some point during the fight Zimmerman felt that his life was in danger, at that point had the right to use deadly force to save his life (it’s called self defense)
It very much DOES matter if Zimmerman started the fight. Florida law states that:
"The justification described in the preceding sections of this chapter [e.g., the self-defense justification] is not available to a person who . . . (2) Initially provokes the use of force against himself or herself, unless . . . [s]uch force is so great that 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."
So, if Zimmerman started the fight (or even if he merely "provoke[d] the use of force against himself"), he was not entitled to use deadly force against Martin in the ensuing fight, unless he reasonably believed he was in imminent danger of death or great bodily harm ("great bodily harm" is typically defined as harm that puts one at a substantial risk of death or permanent serious disfigurement). Moreover, even under these circumstances, if he started the fight, he could not use deadly force unless he had already exhausted all means of escape other than the use of deadly force (e.g., by starting the fight, he would have nullified the "stand your ground" principle, and would have had an obligation to retreat if possible). This is a very difficult standard to meet.
So, yes, it very much matters whether Zimmerman started the fight.
unless . . . [s]uch force is so great that 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.”
As I said, the second Zimmerman felt that his life was in danger he was free to use deadly force. You just proved my point.
Zimmerman was on bottom in the fight according to Zimmerman, witnesses, and the evidence. He yelled for help, and I am sure was trying his hardest to escape. Thus when and if he felt his life was in danger he according to your own posting of the law had the right to defend his own life even if he started the fight.