We know that Trayvon Martin was on top of George Zimmerman, beating him, while Zimmerman was yelling for help. This is supported by witness testimony and physical evidence. Unless you can show that such actions were an appropriate response to something Zimmerman did, then Martins' actions are, by definition, criminal.
I read that while Martin was on top of Zimmerman, wailing on him, Zimmerman’s shirt lifted up exposing his gun. Martin went for the gun and in the struggle, Zimmerman was able to get to it and shoot Martin in the chest. Case closed.