One gray spring afternoon last year, thousands of people descended on Manhattan's Union Square for a rally to call for the arrest of George Zimmerman, the man who shot and killed Trayvon Martin. It had been several weeks since Martin, an unarmed black 17-year-old, was killed by Zimmerman, then 28, who identifies himself as Hispanic, after a confrontation one Sunday night in a gated housing community where both Zimmerman and Martin's father resided.