George Zimmerman's elimination of Trayvon Martin

A comment on the George Zimmerman "innocent" verdict.I get the "reasonable doubt" on whether or not George might have thought he was acting in self defense. But I don't see the total exoneration aspect. Perhaps the issue of the flawed "stand your ground" law in FL and Zimmerman's use/abuse? of it should not have resulted in maximum penalties for George. But his wanton ignoring of directions from police to stand down or not pursue Martin should at least have resulted in a manslaughter conviction. Whether the final struggle was "self-defense" would have been avoided had he done as he was told. That being said, I also don't get those who are now comtemplating a federal civil rights charge against George. How can federal prosecution be used to trump protections against double jeopardy?