As with the battle against Zerahemnah in Alma 45, there were those who were willing to lay down their weapons and those who were not. Moroni does not execute those who still are willing to fight, but rather causes them to be disarmed and taken as bound prisoners. While all prisoners might have been bound, the earlier action of Moroni when he allowed those of the army of Zerahmnah to leave if they covenanted to not fight again suggests that he may have accepted an oath of those who surrendered, and not required that they be bound.