Well there is a lot more to it than that. Whether he provoked the incident has some bearing, even if it doesn't rule out self defense. Obviously there is some injustice if I just start a fight and then, reasonably fearing for my safety, shoot the person.
Further, there is the question of proportionality. If someone is slamming your head into a curb after you approached them to ask what they were doing then you reasonably fear for your life enough to shoot. On the other hand if a person I started a fight with is just punching me and I fall down cutting my head open, and they bend down to lift me back to my feet, I might be in danger of a concussion but shooting a person is likely a step too far.
Self defense is not a blank check to use whatever force an individual chooses to stop the danger present to them. You seem to have come to one conclusion given your partial view of the evidence and judged that Trayvon Martin was committing aggravated assault which necessitated Zimmerman using lethal force. But there are two people who get the presumption of innocence here. The special prosecutor had a view to much more evidence than all of us, and has come to the conclusion that a trial is prudent. Lets hope she didn't base her decision on politics, at the very least.
