This is speculation on my part, but based on the facts above being ignored by the media frenzy, this is what I think happened:
Zimmerman was belligerent, possibly even drunk and certainly didn't talk to Martin as respectfully as the TSA talks to Muslims. He may have even said something racial like what is a guy like Martin doing walking in their gated community.
Martin was possibly even drunk as well, but not with alcohol. I'd say he was drunk with pride. Pride which wouldn't allow a racial inferior, particularly a pudgy white guy to disrespect him. He felt he had not only a right, but a duty to teach this cracker a lesson particularly when Zimmerman followed him and pushed the envelope.
What he didn't know, however, was that Zimmerman was carrying an equalizer and was unwilling to submit the beating which he so richly deserved for his disrespect. That's when the real trouble started.
My mama, like so many others, taught me that "sticks and stones may break your bones, but names will never hurt you." The gangsta culture, OTOH, teaches that a beating is a perfectly proper response to being disrespected or insulted and said beating may escalate to even lethal force if the insult or disrespect is sufficiently offensive.