We have to keep in mind that the Nephite leaders knew that in the future centuries the prophets had said the Lamanites had a greater promise of survival than the Nephites. Perhaps this might have been a factor as Moronihah made his decision to send the Lamanites back to their own country in peace.
“Caused That the Lamanites Who Had Been Taken Prisoners Should Depart Out of the Land in Peace”
Not only were the Nephites back in possession of Zarahemla, but the Lamanites stood around in helpless bewilderment. Coriantumr was now dead, and Moronihah had to make one of the most difficult decisions of his entire life. What would he do with this host of Lamanite survivors? For reasons best known only to himself, here is what Moronihah did.