It is a tragedy that Trayvon Martin ended up dead in his scuffle with George Zimmerman, a tragedy that Zimmerman caused. He shouldn't have assumed that Martin was up to no good, and he shouldn't have pursued him after a police dispatcher warned him not to.
And yet not every tragedy or bad judgment is proof of a crime, much less a federal civil rights violation. When federal prosecutors bring charges after defendants have been acquitted in state court, they test the principle of double jeopardy, forcing suspects to stand trial twice on essentially the same facts. Even when technically acceptable, that offends the general notion that a defendant should have to answer only once for an alleged crime, and it's why federal authorities should use that power sparingly and only where an obvious federal interest is implicated.
The prosecution of the Los Angeles police officers who beat Rodney G. King, for instance, was justified by the clear federal interest in protecting the constitutional right to be safe from harm in police custody. The convictions of two officers in that case were justified and welcomed. Here, however, there is no reason to believe that federal prosecutors would fare better than the state of Florida did, and there is no clear federal right to vindicate. Unless federal authorities uncover some new piece of evidence that suggests obvious racial animus in Zimmerman's actions, he should not be prosecuted again.