No forget that, I'm not done here. A trial is conducted based off the evidence put out by either of the parties. If the evidence clearly leans to Zimmerman(said by Kaiiu himself) then there isn't anything to argue over. No further evidence has been brought forward, end of story.
Furthermore, you can't try someone based on evidence not found. while Martin didn't have DNA on his knuckles, it was clearly stated multiple times during the trial that the evidence collection by the police and forensics was incredibly sloppy and botched.
Lastly, with the police ruining evidence as such, we're forced to base it off the story of panicky witnesses, Zimmerman himself, and the few and far between pieces that lead to Martin being cast as the aggressor such as him not having much damage besides the gun shot, the scuffed hands, Zimmerman's physical state on police tapes and mug shots.