If they wanted to kill the hostages, they would have just killed them instead of taking them hostage. The whole point of a hostage is that it increases your power at the negotiating table. It gives you leverage. The hostage is something to trade in exchange for concessions, or the threat of harm to force the opponent’s cooperation. Neither of those work if the hostage has been killed or harmed, because any bargaining power you would have had from holding them goes right out the window as soon as the hostage is killed.