Contrary to a common discussion point, Florida's controversial "stand your ground law" was never an issue in the Zimmerman case. Rather Zimmerman offered a routine self-defense claim, in particular that he shot Trayvon Martin when Martin was on top of him hitting him and shoving his head against the pavement. Zimmerman offered up an expert who said that story was consistent with the physical evidence.
While under Florida law, self-defense is an affirmative defense, once the defendant presents evidence of self-defense, it becomes the state's requirement to prove beyond a reasonable doubt that Zimmerman did not shoot Martin in self-defense.
Here is an article which talks about how the burden of proof on self-defense in Florida has been clarified in recent years.
People who try to turn the failed prosecution into a racial thing are terribly misguided. The bottom line is the only person alive who knows for sure what happened that night is George Zimmerman. His story that he shot Trayvon Martin in self-defense simply could not be disproved beyond a reasonable doubt. It appears that the prosecution never had a strong case and probably only filed criminal charges in response to political pressure, including pressure from the White House.
In a subsequent wrongful death action, the burden of proof is a "preponderance of the evidence" which means that Martin need only tip the scale in his direction. Under that standard, Martin's family has a much better chance of prevailing.