Stand your Ground may have weaknesses but that is not relevant here since that is not the defense Zimmerman used.
You are right that this incident was more the tragic outcome of collective stupidity than racism. Zimmerman was an idiot for getting out of his vehicle when he should have just reported and left. And Travyon was an idiot for not just heading straight home when he realized he was being followed. According to the evidence he had 4 minutes when Zimmerman lost track of him and he could have reached home in minutes. For some reason he decided to stay and unfortunately there is no proof of specifically who started what though there is pretty good proof that Zimmerman was getting the worst of it till he used his firearm.
If there were laws against stupidity they would both be in jail if Travyon was still alive. Legally there is no proof of who started the fight and that is why Zimmerman got off. If Travyon shot Zimmerman instead he would have got of free for exactly the same reason, he could show he was in fear of his life and no one would be able to prove otherwise.