You're assuming that Zimmerman, who was following Martin around in his vehicle for long enough that Martin noted it during a phonecall and said his reaction was going to be to "walk faster," got out of his car and politely asked a question. That all remains to be determined, since despite the title of this thread NO ONE SAW WHAT HAPPENED AT THAT POINT. No one witnessed the initiation of force. The witness, being quoted all over these forums as proof of Martin's "thug" nature, saw Martin on top at one point, and punching, and then the next time he saw the two of them, Martin was dead. It is interesting how Martin being on top signals that he's a monstrous, malicious thug... but Zimmerman having shot him does not signify the same. It appears to be a product of the order of the two images.
Prior to that, would it change your mind if Zimmerman were grabbing Martin's hoodie? Would it change your mind if Martin, reacting to the witness' own advice to "Stop" because he was "calling the cops," caused Martin to stop punching, at which point Zimmerman shot him? Or perhaps it's as people say, and Martin viciously attacked someone who had been following him. Maybe Zimmerman made a comment Martin didn't like. Maybe Martin is an evil, drug-crazed maniac as Zimmerman suspected. Who knows?
But the bottom line here is that the cops really didn't know, either, and decided Zimmerman's story was 100% accurate and true, which is why he was not arrested.
This does go to the grand jury on April 10th. Zimmerman just gets to be out of jail in the meantime.