Blame New York’s progressive leaders for the horrific filmed murder of a 14-year-old boy on a Bronx playground this week. Late Tuesday, a mob of teens jumped ninth-grader Angel Mendoza at the Williamsbridge Oval Playground — pistol-whipping, beating and stabbing him to death while they recorded the stomach-churning ordeal on a cellphone camera. Angel was hanging out with a boy who’d been involved in a earlier fight with the group, and the sadistic teens killed him just for being in the wrong place at the wrong time. His stomach-churning death is an especially brutal one, but far from unique. These...