The case of George Zimmerman has always been morally ambiguous, but as a matter of law the jury's vote to acquit was quite predictable. When a defendant claims self-defense, as did Zimmerman, Florida law stipulates that it is the state's obligation to prove, beyond a reasonable doubt, that the defendant did not act in self defense. This is a high bar in the best of circumstances -- as the prosecution's best witness is invariable dead -- but the Zimmerman-Martin altercation offered very challenging circumstances. There was little ambient light, and there were no reliable eyewitnesses.
In court, a jury is commonly asked to "determine the facts of the case," which is a bit silly in a situation such as this. The facts are undeterminable. There is no prospect of arriving at any certain conclusion. The jury runs the risk of making two types of error: letting a guilty man go free, or convicting an innocent one. In the American system of justice, we fear the latter type of error the most. Hence the defendant always receives the benefit of the doubt; when the jury cannot make a firm conclusion one way or the other, acquittal is the default option. Juries do not find defendants "innocent," they find them "not guilty." It might be more accurate to say "not proved guilty."
There is understandable consternation about Florida's "Stand Your Ground" law, one of the nation's most permissive regarding self-defense claims. It might make sense to repeal that law -- to effectively require would-be self-defenders to retreat rather than kill if retreat is an option. But in the Zimmerman case, "Stand Your Ground" was largely a red herring. The inevitability of acquittal was rooted in the circumstances of the case. Suppose the state could have won conviction by proving that Zimmerman had an opportunity to flee. How would they have proved that case?
What would really change the dynamic in self-defense cases would be to require defendants to affirmatively prove that they acted in self-defense -- to take the benefit of the doubt away from them. This would in no way remove the essential ambiguity of the Zimmerman-Martin case, but it would definitely result in more convictions. If that's what we want to happen.