It doesn’t matter if Zimmerman kept following Martin after the dispatcher told him not to (he is a free citizen and can walk down the street if he so chooses)
It doesn’t matter if Zimmerman asked Martin what he was doing(he is a free citizen and can talk to whomever he so chooses, it’s called free speech)
It doesn’t even matter if Zimmerman started the fight.
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)