Christ was not "immortal" when He first came to earth; his mortal mother's contribution to his physical body ensured that. He did die, spent time in the spirit world, and resurrected with a perfected, immortal body. The power to deny death's permanence he received from His Father. The shooter's free will was the cause of the suffering of his victims. But the tragedy was in his sin, not in their deaths. He will have to pay for his sins, and his victims have been ushered into the presence of the Father, there to live in glory with Him until later, when they will have the opportunity to be reunited with their families.