1. Zimmerman's Gun was infact ILLEGAL because he was a convicted felon.
2. Trayvon Martin DID NOT provoke Zimmerman to pursue him. Zimmerman did that on his own motivation, taking his illegally possessed weapon with him.
http://en.wikipedia.org/wiki/Stand-your-ground_law
In the United States of America, stand-your-ground law states that a person may justifiably use force in self-defense when there is reasonable belief of an unlawful threat, without an obligation to retreat first.
So, lets asses the situation.
Zimmerman followed and possibly confronted him. Trayvon had the right to stand his ground until an officer arrived to trespass him after/without verbal confrontation WITHOUT THE ILLEGAL GUN.
Instead, things escalated. Both could have been charged with assault (It doesn't matter who the aggressor is. If they both duke it out without taking an opportunity to retreat, it's mutual assault)
Things were escalated further when the illegal weapon was brought into the conflict.
Zimmerman shot and killed Martin. Stand your ground or not, the weapon was illegally in the possesion of Zimmerman because it was unregistered, unlicensed, and because he was a convicted felon. Therefore, it is illegal use which does not fall under the stand your ground law.
What does it mean since the Stand your ground law doesn't apply? It means that he committed murder.