I find it rather surprising that Trayvon Martin's history of fighting was excluded as evidence in the George Zimmerman trial.
Zimmerman was underneath Martin (witnesses), he was getting badly beaten (injuries), he seems to have been shouting for help (contested). He drew his gun as a final recourse believing himself to be acting in self-defence.
This is all kinda coherent.
The fact that the result was a death doesn't make the dead person innocent.