This is a somewhat complex case. It seems that the lack of clear evidence as to exactly what happened is the reason that Zimmerman was set free. It is a tragedy, as a life was lost, and suffering must be endured by relatives and friends.
To punish an individual by imprisonment, without clear evidence giving cause for, and validation for, that punishment, would also be a tragedy. Perhaps I've missed some key points of evidence. Does anyone believe that the evidence is clear as to exactly what happened? Otherwise, all this controversy, including the civil rights aspect, is really about something other than the specifics of this case.