If Zimmerman wanted to shoot him straight out, I'm sure he had the opportunity. It's not what this was about...and I'd sure like to know what Tray had in his pockets.
If Martin had managed to kill Zimmerman instead, would he have been caught? There were witnesses, but did anyone see him well enough to give a description to the police (who were already on their way)? If not, he could have gone back to Miami and possibly gotten away with it...at least until he bragged one time too many about killing a cracker.