Clearly it doesn't given the verdict, doesn't change the situation, toting a gun is not part of Neighborhood watch protocol for a reason. He was negligent on procedure. His overzealous actions ignored his obligations as a Neighborhood watch representative and a kid is dead because of it.
I've given you the best explanation I can for your original question.
It was a breach of procedure, resulting in a death due to his negligence.
That is why someone can't in all honesty represent Neighborhood Watch and go about carrying a weapon. Or at least this result is what is trying to be avoided.
More to the point, the resulting confidence Zimmerman got with his loaded weapon is why he breach procedure and left the standard protocol of observe & report & stay put until police arrive. Without his safety net(gun) he in all likely hood wouldn't have continued to pursue Trayvon, which would have likely ended this whole debacle and Trayvon would still be around to live.