Race aside, it was ALWAYS going to be a basically impossible conviction. This is one of those cases -- and police encounter them every day -- in which everyone knows what really happened but you just cannot prove it. The only thing they had going for them were Zimmerman's many often contradictory statements which cast doubt on his credibility, but ultimately all anyone really knows is that Zimmerman followed him, they eventually met and a fight broke out, Zimmerman was slightly injured and Martin was killed.
Now as I said, I believe Zimmerman actually committed murder one. I believe this based on the 911 tape. If you listen to this, and know what you are listening for, it is nothing more than Zimmerman reciting a checklist for a "justified" shoot. Anyone who has had some law enforcement training would know this list. Zimmerman was setting the entire thing up right there, and in my opinion he was PLANNING on blowing Martin away right there on the street, with the 911 operator in the background. But Martin ran.
Based on their questions in follow up interviews I believe the law enforcement shares this opinion.
In my opinion, Zimmerman is a sociopath. Note as well how eager he was to share his wildly varying stories. He's comforatable lying his ass off -- he invents his own reality as he goes along, and doesn't feel even the slightest concern when that invented reality fails to match the physical evidence in front of him or the stories he has told in the past.
But there it is. Can you prove it? Nope. Can you prove that Zimmerman wasn't jumped by Martin? Nope -- it's ludicrous, it's obviously unbelievable, but you can't prove it. And since you cannot prove it, that's it. There's nothing there. All the following and stalking and running, all the stories and fanciful claims, all of that goes out the window.