Last week, in Chicago, 16-year-old Darryl Green was found dead in the yard of an abandoned home. He was killed, relatives reported, because he refused to join a gang. Unlike most tragedies, however—which remain local news—this one caught the attention of conservative activist Ben Shapiro, an editor for Breitbart News. Using the hashtag “#justicefordarryl,” Shaprio tweeted and publicized the details of Green’s murder. But this wasn’t a call for help and assistance for Green’s family, rather, it was his response to wide outrage over Saturday’s decision in the case of George Zimmerman, where a Florida jury judged him “not guilty”...