WASHINGTON — Leaders of the National Rifle Association on Thursday accused gun control advocates of exploiting the deadly Florida school shooting, striking a defiant tone amid a renewed debate over guns and school safety. NRA Executive Vice President Wayne LaPierre, in his first public comments since the shooting in Parkland, Fla., said NRA members mourn for the Florida victims but at the same time issued a searing indictment of opponents of gun rights for attempting to “exploit tragedy for political gain.” “They hate the NRA.