You can shoot someone if your life or someone’s life is threatened. That is the law as I understand it in FL.
So a stranger could have shot Martin if they had come across him smashing Zimmerman’s head into the pavement.
An individual in Florida who legally carries a concealed weapon can legally use his weapon to stop a felony in process.
That makes sense to me. But then I don’t know FL law (except as it has been applied in this trial).