Second degree murder is defined this way:
second degree murder n. a non-premeditated killing, resulting from an assault in which death of the victim was a distinct possibility. Second degree murder is different from First Degree Murder which is a premeditated, intentional killing, or results from a vicious crime such as arson, rape, or armed robbery. Exact distinctions on degree vary by state. (See: murder, first degree murder, manslaughter)
It sucks. It seems Zimmerman should be charged with something, because as I've written in the past, if I were seventeen again, and I knew some strange man was following me down the street, I'd either knock on the nearest door and get the person to call 911, or I'd turn and fight. Being young and hot-headed, I probably would have confronted the guy to make him think I'm not worth following anymore. So maybe Trayvon Martin did start the violent conflict, but what the hell was Zimmerman doing following him? Who the hell is he to confront someone walking down the street? If you're in a neighborhood watch, you are there to observe and report, not stop people who are walking down the street because you think they look suspicious to you.
I'm not sure the charges fit. I want them to fit, as I'm sure many other people do--we do know that it is true--beyond a reasonable doubt--that George Zimmerman killed Trayvon Martin. It does seem that he should be charged with something, though. Manslaughter. Stalking. Vigilantism--he was trying to replace the police by following Martin himself. Something. I consider Zimmerman's behavior to be dangerous to society, and I don't think he should just walk free, knowing that he definitely killed someone.