Last Thursday a 25-year-old criminal with a long history of arrests named Vincent Pinkney went on a stabbing spree in Manhattan. His first victim was an Italian Ph.D. student at Columbia University:The student, Davide Giri, was traveling home from soccer practice just before 11 p.m. when he was stabbed in the abdomen about two blocks from his apartment building, the police and friends said. He was taken to an area hospital, where he was pronounced dead…Mr. Giri, a Ph.D. candidate in computer science who was expected to graduate in May, had begun working as a research assistant at Columbia around...