The question is not whether Zimmerman was acting like an idiot, or whether Martin looked like a thug: The question is who attacked whom in a physical way. Insults, screaming, etc. do not justify battery. Even the most heinous crime in our country, hurting someone's feelings, does not justify violence.
So if Martin took the fight to Zimmerman, Zimmerman would be justified in defending himself. Likewise, if Zimmerman attacked Martin, he would not.
+1. Though all this is only really useful as a theoretical exercise. To me, the pertinent question is not what happened (since we will never know), but what happens now. A jury will convict or acquit and we all have to live with the consequences in our own neighborhoods. To me the question is, can we do anything to prevent the media from blowing this up into riots or, during an already extremely tense and racially charged election year in this country, some kind of race war. Some of the sensationalist crap I've read about this makes me think they would like nothing better. I just imagine people in offices somewhere looking over the 1968 circulation numbers, salivating over the possibilities.
The other bad effect, which has been mentioned, is the chilling effect on people trying to keep their neighbors and friends safe. I personally appreciate that some people are willing to put themselves at a little risk in order to try to keep their neighbors safe from harm. Sometimes those are police, but they can't be everywhere (nor should we want them to be), and they are rarely there when they are needed (such as when two men were burglarizing a nearby residence, occupied by a woman and her children). If all of us just decide never to stick our neck out for anyone else, because we might end up having to defend ourselves in the process (regardless of what exactly happened here), then everyone is on his own. That's not much of a community.
Last thing I don't see mentioned a lot is that if one is armed, every altercation can go to a level of deadly force. Maybe (as GT thinks) if Z hadn't been armed, he wouldn't have felt comfortable checking out the situation. OTOH, maybe he still would have felt a sense of duty to his neighbors and checked it out anyway, and gotten his head beaten (further) into the concrete for his trouble, and died. There is no way to know. Keep in mind, his intent that night, from all evidence, was to help keep his neighbors, including T's family, safe from harm. I guess it's another argument against the isolation most people live in where people don't know any of their neighbors. If T and Z had recognized each other, this never would have happened.