As I understand it, the basic demographic groups that gave Obama his greatest support in 2008 are operating at lower levels of enthusiasm than expected at this point in the 2012 election cycle. The blacks, the women and the young need (from a Dem viewpoint) a big kick in the you know what to re-energize them and get them out to vote in November. The Zimmerman case was "chosen" for that purpose.
Since the boat has more holes in it than the Titanic, it will be forgotten soon.
But that's OK.
Even though it is very rare, eventually a non-minority race member will kill a minority race member in self-defense and they can start over again.