In U.S. common law in most jurisdictions, a citizen may make an arrest provided he or she sees the crime committed, and to use force to hold the suspect until police arrive. If the citizen making the arrest is mistaken, there are no protections for that citizen and he or she can even be prosecuted for false imprisonment or even kidnapping, both felonies.
Trayvon Martin was not committing a felony nor had any felony been committed in that area that evening at the time of his murder. George Zimmerman, already in violation of Neighborhood Watch procedure by being armed, was explicitly instructed by the police not to engage Martin and did anyway. Since no felony had been committed, Zimmerman had no authority under citizen’s arrest to stop, much less detain, Martin and was therefore committing felony unlawful imprisonment, kidnapping in this case since he was armed.
Trayvon Martin's death occurred not only during but as a direct result of said felony. George Zimmerman is therefore guilty of felony murder. At least in the realm of objective reality.
Under U.S. law, no one can be tried twice for the same crime, and a verdict of innocence, unlike a verdict of guilty, cannot be appealed. Zimmerman could still be tried for civil rights violations, however, like the officers in the Rodney King case. One thing that really bothers me about this trial is the fact that it had only six jurors as opposed to the usual twelve, and the fact that they were all middle-class white women (it would bother me if they were all men or all black or or all working class or all wealthy Perelandran hermaphrodite atheists).
By the way it handled the case, it seems upholding Florida’s “Stand Your Ground” law is much more important to that state than providing justice for one of its murdered citizens and his family, unless the defendant happens to be black and female (Marissa Alexander) firing a warning shot to scare off her abusive husband (she is sentenced to 20 years).