My first thought would be fear of more gun control (i.e. repealing SYG laws) though the sequence of events seem to suggest that Zimmerman may not have acted in accordance with SYG. Whatever else comes out of an investigation of this matter, it seems that Zimmerman was intent on pursuing Martin for some reason and acted contrary to the advice provided by 911. I suspect that what right-wingers are really afraid of is that President Obama (and African-Americans in general) are looking for "payback" against whites and now, since President Obama was elected in 2008, they have the authority to exact it, whether that's in terms of repressing whites (using "government" to take away their "freedoms"
and/or demanding reparations (i.e. taking their money and giving it to undeserving minorities). I think that, ultimately, defending Zimmerman and smearing Martin and trying to eliminate any sympathy for him is more tribal- defending whites against minorities whom they believe are out to "get them". 
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