Twenty-year-old Davonte Flennoy was killed on a Monday night in June 2012. As the setting for a murder, South Rockwell Street, in Chicago’s Marquette Park, was familiar enough to border on cliché. The last thing Flennoy saw as bullets entered his chest, arm, and head could have been the dumpsters artlessly ranked in the alley behind him or the razor wire curling atop a nearby chain-link fence. More than fifty other people had been shot throughout Chicago in the preceding ninety-six hours, ten of them fatally. Most of the shootings, and every one of the murders, occurred on streets like…