If Zimmerman is not lying then the reason Trayyvon Martin is dead is that Martin acted in self-defense. You might even say that he stood his ground.
Being stalked by a complete stranger who nearly twice your size for no apparent reason is extremely reasonable grounds for believing one is in danger. (The reason given by Zimmerman being ludicrous.) If Martin reasonably thought he was in danger then had two options: flee or stand his ground and fight for his life. That being said, turning on a stalker usually not the smartest move, but then again the Martin was both scared and a kid.
Martin, like Zimmerman, had the right to self defense. But unlike Zimmerman, Martin did not seek trouble. Trouble came to him. If Zimmerman did not want the person he was stalking to turn on him, he should not not stalked him in the first place.
Of course this all assumes that Zimmerman has been completely truthful which is far from clear.