Prime Minister David Cameron says the "completely unforgivable" slaying of a British hostage in Syria demonstrates the necessity of destroying the Islamic State extremist group. ** appeals made any difference," Cameron said Saturday after receiving a security briefing from Foreign ******, intelligence and military officials at Chequers, his official country retreat on the IS killing of English aid worker Alan Henning. When asked whether he believed the extremists would kill more of their Western hostages, Cameron suggested the only way to stop them was through military action.