Although the Lamanites had surrendered the city, they hadn’t completely surrendered their hope of escape. Thus, they would attempt to break free, causing more fighting, such that two thousand of them were killed “after they had surrendered themselves prisoners of war.”
This created a problem that had to be solved. As Helaman noted, they needed to either take the prisoners to a place where there were more men to guard them, or they needed to kill them. Perhaps in accordance with the principles Moroni espoused, keeping them prisoners was preferred to killing them. The decision was made to send them to the land of Zarahemla, as had been previously done with Lamanite prisoners (see verse 11).