Ok, wait a minute... GZ obviously had no intention of killing TM because as soon as the shot fired he pushed him off and got up and asked the neighbor to help restrain him. If there was premeditation to kill, he would have shot more than once.
Plus, remember, he was surprised that TM was dead the next day.
The gun jammed. But I am of the opinion that if Zimmerman had been able to get out from under Trayvon and retreat without shooting, he would have.
Use of lethal force for self-defense is reserved for "kill or be killed" situations. Any firearms instructor will tell you to aim for "center mass" - the heart. This means your intent is to kiil your attacker. You may not accomplish that, but at the moment you fire that should be your intent. If you fire with only the intent to wound that wil be taken as evidence that you did not believe it was a "kill or be killed" situation.