Sorry I'm missing your point. If it's shown Martin has powder burns on his hands, what does that tell us other than what we already know, that they were fighting when the shooting happened?
That would mean that the gun was out, in play, before the witness known as “JOHN” saw the scuffle on the ground ~ this guy is not a cop!
We don’t know when Zimmerman had his gun out. If Zimmerman had it out at the time it got physical, it would make sense for Martin to want to beat Zimmerman, because he would have thought Zimmerman was planning to shoot him.