I've stayed out of this particular controversy up until now, since, based on quite a bit of associated knowledge, I can generally say that most shootings do not involve a "good guy" and a "bad guy"... usually they both are shady to some degree. But that's not what caught my eye in your post.
You are asserting that you cannot start a fight and claim self-defense (as an attack on Zimmerman, I assume... it will be interesting to see what you will say if it is established that Martin jumped Zimmerman). But there is a corollary: you cannot continue a fight beyond reasonable means either.
True story: a friend of mine in high school had a bully who was threatening him in the school. It was because the bully thought that my friend was hitting on the bully's girlfriend (the truth was that the girl spent more time on her back than a luge driver... and was hitting on my friend... along with every other male in the school. But that's neither here nor there).
After school one day, the bully followed my friend out of school and across the street to a McDonalds. In the parking lot, the bully attempted to start a fight. My friend turn around and walked away, at which point the bully jumped on his back from behind. After about a minute of struggling, my friend managed to get the bully far enough around his body so that he could lunge forward and slam the guy's head into a nearby parked car. This stunned the bully, so my friend repeated the maneuver about five more times, leaving pronounced dents in the car and the bully dazed on the pavement.
When the police arrived, my friend told his story to the police. BOTH were arrested and charged with assault. Why was my friend charged? According to the statutes in the state of VA, my friend could respond defensively to the attack up to the point where he was able to disengage and was not under direct assault (after the first head slam). The fact that he continued to slam the bully's head AFTER he got the upper hand was now an assault!
So, assume that Zimmerman walked up to Martin, called him the "N" word, spit in his face, and grabbed him by the testicles. Martin would have had every right to lash out in self defense UNTIL HE COULD DISENGAGE (since the stand-your-ground law does not apply here, according to the law's author). However, sitting on top of Zimmerman and pummeling him (as witnesses describe and as physical evidence supports) immediately transforms Martin from the victim to the aggressor (in the eyes of the law), at which point Zimmerman, even if he initiated the conflict, could then act in his OWN self defense (once again ignoring stand-your-ground, since it doesn't apply).
If Martin was alive today, he could (and probably would) be charged with assault, since his aggressive actions at the end of the fight went beyond what was necessary to protect himself. Just like you can't chase a guy down the street and shoot him, you can't sit on a guy and pummel him after you have the upper hand. That's the law (whether right or wrong).