America loves paranoia.
We're a country fascinated with ghosts and the paranormal, mind-reading and angels. A long streak of extensive conspiracy theories dots our national landscape: the JFK assassination, Obama's birth certificate, the "real story" behind 9/11, Freemasons, a dozen different Jewish plots, and so on. Richard Hofstadter wrote about this nearly forty years ago, and it bears repeating: in America, paranoia is gold.
When you look at Trayvon Martin's killing, it's easy to simply say that George Zimmerman was racist, his actions were motivated by racial fears, and the subsequent police response stems from a wholehearted indifference to the life of a black teenager. I'd argue a lot of that's probably true, but as Ta-Nehisi says, "[T]his is about race along with..."
That "along with" is paranoia. In this case, it's the sort of paranoia that leads to Florida's rather expansive Castle doctrine, which has the rather delightful effect of making "anywhere you have a right to be" your castle. It means that every legally armed individual retains a measure of protection against responsibility for shooting and killing another person, expanded by the scope and scale of their own personal paranoia.
George Zimmerman managed to call the police department 46 times in the past 14 months to report various crimes. The two explanations for this are that he's the unluckiest man on the face of the planet, doomed to forever witness crime but never stop it (until now), or that he's a paranoid vigilante with a fixation on black men who got sick of having the police do nothing about his repeated, meritless complaints.
I'd bet on the latter, but the InTrade value on the former is really great right now.
An expansive self-defense doctrine turns the expression of paranoid activity into a socially acceptable, excused form of vigilantism. Hunting down and murdering a teenager visiting his father because he "looked suspiciously at houses" and "walked slowly" - teenagers being known, of course, for their otherwise purposeful, focused strides - becomes a he said, he dead proposition. The paranoiac who fixates on black youth is protected, because feeding a certain form of majoritarian paranoia bears rather awesome political fruit.
The issue is not simply that a non-black man has a problem with the existence of young black men. It's that the law in Florida is structured in such a way that the former can use the latter for target practice, and says nothing so long as one is afraid of 140-pound teenagers for the right reasons.