Trayvon Martin & Self Defense

Just wondering: why is it that self defense was invoked on behalf of George Zimmerman and not on behalf of Trayvon Martin? Martin was peacefully walking home from the convenience store with his bag of Skittles, chatting on the cell phone with his friend, when he realized, accurately,that he was being followed, i. e. stalked, by a stranger who was not a uniformed police officer. Nor was the stalker in a police vehicle. The stalker parked his car and came after Martin without identifying himself and, as it turned out, he was armed with a deadly weapon. In these circumstances, wasn't Martin entitled to defend himself, to "stand his ground?" If so, i. e., if he was acting lawfully in self defense against an armed stalker, how could it be legal for Zimmerman to kill him? Just wondering?