For Valerie Clark, the breaking point was when a stranger threatened her outside her home. It was about two weeks ago that the man came up to her as she was entering her apartment building on the Upper West Side. “You’re fixing to get attacked,” he growled. “I know where you live, and you deserve it.” Clark skirted him, called the police and made a vow: “I said, ‘I’m not going to turn my back.’” So the 47-year-old, whose daughters are 12 and 14, has spent the past two weeks patrolling her neighborhood with the Guardian Angels, the volunteer organization founded...