So by that logic, if a police officer pulls you over and walks right next to your window so that his holster is within the range of your arm, he is then within his rights to shoot you dead, because simply by being in proximity to a firearm, you are armed.
If there are two cops, and you grab for the gun of one; you should expect to be dead.
You're not suggesting that Zimmerman walked up to Martin, pulled out his gun and shot Martin are you?
No, that's what you were suggesting.
Zimmerman walked up to Martin, the gun was within Martin's reach, Martin wasn't "necessarily unarmed" regardless of his age, so Zimmerman shot Martin in self-defense.