From the Sentinel article Cheese posted:
Justifiable use of force
Doesn't say feel. They could arrest if they don't think he was thinking reasonably. The judge/jury could decide.
Now 776.013 says this:
So for the sake of argument, let's just say Zimmerman is following Martin. If he is in a place where he has a right to be and Martin feels threatened and attacks him out of fear then according my reading he would be justified in shooting. Not that I think that's what happened. But if Zimmerman claims this happened and no one can dispute it, what can be done? There is still no report on what Zimmerman says happened between his 911 call and the fight.
Some side observations on the article about Martin and Zimmerman:
The football photos of Martin are apparently not current, since it says he played ball in his early teens. That would explain why he doesn't look like a 17 year old in the photos.
Ten day suspension for excessive tardiness? So I guess the schools think if you can't be on time, don't come at all? Florida: Tough on Tardiness. Shooting? Not so much.