Police shot and killed a 20-year-old man in an exchange of gunfire Thursday night after he allegedly went on a gun-toting tirade around the Upper East Side and threatened to shoot up Mount Sinai Hospital, officials said. His reign of terror began around 7 p.m. when he brandished a firearm on an elevator of a residential building on Madison Avenue between East 106th Street and East 107th Street, NYPD Chief of Patrol Philip Rivera said in a press conference late Thursday.