There is released evidence that Zimmerman was shouting for help before Martin was shot. In addition, the house Martin was walking around had been burglarized recently, as had a number of other houses in the neighborhood. Furthermore, there were over 400 registered calls to the police from the neighborhood in question from residents, 9 burglaries, 1 shooting, and numerous reported attempted burglaries. It was also reported that these crimes and attempted crimes had created an atmosphere of fear in the local community.
With this in mind, it is not unreasonable that anyone in the neighborhood watch would be a bit jumpy, more so if yet more trespassing had just been observed. If he was not participating in unlawful activity and was in fear of great bodily harm, which several pieces of evidence point towards, then he was entirely within his rights to fire his weapon defensively.
Regardless of whatever story the mainstream media has been pushing, this is not necessarily a case based on race.
On the subject of the law, here is a portion of it from
http://www.leg.state.fl.us/statutes/index.cfm?App_mode=Display_Statute&Search_String&URL=0700-0799/0776/Sections/0776.013.html.
...(3) A person who is not engaged in an unlawful activity and who is attacked in any other place where he or she has a right to be has no duty to retreat and has the right to stand his or her ground and meet force with force, including deadly force if he or she reasonably believes it is necessary to do so to prevent death or great bodily harm to himself or herself or another or to prevent the commission of a forcible felony.
One would think that having one's head bashed into the sidewalk could possibly cause great bodily harm. There is no substantial evidence showing that Zimmerman was in fact engaging in unlawful behavior prior to the shooting, but instead inflated or in some case fabricated claims by the media, often making the fallacy of appealing to emotional outrage rather than fact or reason.
Do remember that much if not all of the televised mainstream media in America is classified as "entertainment", not "news".
tl;dr: This is a legal problem, not a racial one.