As far as I'm concerned, it should all come down to when the law (if any) was first broken, and by whom.
We can sit back and argue what we would have and/or wouldn't have done in such a situation, but the only thing that should matter is if Zimmerman actually broke any laws, and if so, when. He was charged with a crime, but I'm still unsure which law the prosecutor believes she can prove Zimmerman actually broke.
In the end, I don't think the prosecution has enough evidence to get a conviction. I believe the only witnesses who could possibly give an accurate account of what went down and how are Martin and Zimmerman; and Martin is unable to testify at the moment. I don't believe there is another witness out there that can honestly say, without reasonable doubt, how things went down that night.
I truly believe that the prosecutor decided to arrest Zimmerman because she felt like she had no choice. If they hadn't, the entire state of Florida would be in ruins, along with God knows how many other parts of the country. This way, at least they can say they tried, but the jury had the final say.