LAS VEGAS — Rob Ledbetter's battlefield instincts kicked in quickly as bullets rained overhead.The 42-year-old U.S. Army veteran who served as a sniper in Iraq immediately began tending to the wounded, one of several heroes to emerge from the deadliest mass shooting in modern U.S. history. Amid the massacre in Las Vegas, which left 59 people dead and more than 500 injured, there were acts of compassion and countless heroics that officials say saved scores of lives.