Trayon Martin was not a skinny kid. He was a fairly-ripped football player who talked tough and acted tough. He confronted a stranger who was stalking him. Many of us would do the same, but probably not if we felt physically outmatched. If he didn't throw the first punch he was murdered.
Since no crime had been committed and no reason existed to believe one was about to be committed, I think some degree of manslaughter is the best Zimmerman can hope for even if Martin hit him first which is probably what happened. He stalked him, ignored the dispatcher's instructions to stop following and let the police check it out, eventually alarmed Martin enough to cause a confrontation.
While he may have ultimately shot in self defense, he was the original aggressor and created the situation entirely out of nothing.
The desperate "need" to present this as a one-sided ravial vendetta against a cute, cuddly, defenseless 14 year old (the photo used by the family and 99% of the media) does nothing to further the cause of justice. Neither does looting stores, offering a bounty, making a hate crime out of it.
FWIW, Sanford, FL has a crime rate much higher than the national average:
http://www.cityrating.com/crime-statistics/florida/sanford.html