This is why so many people find this process a farce. It is tribalism at its worst. The threat of race riots and unrest was issued before Zimmerman was arrested and charged. Everything since then has been a negotiation.
"Will you not riot if we at least arrest and charge him?"
"No, but there had better be a guilty verdict for second degree murder!"
"Uhm, sorry, that is isn't looking plausible considering the evidence that is coming out. Will you take manslaughter?"
"Manslaughter? Are you kidding? It better be at least that, and we wont be too happy about it either!"
When the criminal justice system is so susceptible to blanket threats from one group or another, then multi-ethnic democracy is on the inevitable path to sectionalism and Balkanisation.