*sigh* It's not a "Shoot First" law and the "Stand Your Ground" law, as the facts are known, don't apply.
First, SF vs SYG. Stand your ground means you have no duty to run away from an attacker. Suppose a knife wielding maniac is advancing on you...you can shoot him, you don't have to run away (some states require that you attempt to run away before shooting). Or in a home invasion, you can shoot the guy when he breaks through the door, instead of having to retreat to the furthest bedroom (this is also required in some states). There is no such thing as a shoot first law...you make it sound as if you can just pop anyone you don't like.
In this case, as far as we know, Zimmerman was already being attacked and taken to the ground after turning to go back to his vehicle. Most of the states in this country would allow you to use deadly force to protect yourself if someone is on top of you and bashing your head into the ground. It has nothing to do with the stand your ground laws and everything to do with self defense.