The prisoners being escorted to Zarahemla had already proven rebellious, and they revolt again. This time many are killed, but many also break through the ranks. Of course Gid reports that they killed more than broke through, and this may have been correct. It would not be surprising, however, if the escapees were more numerous than the dead, and Gid simply reported it in the most favorable way. The end result was that the Nephites lost their prisoners, and some of them were able to escape and make their way back to the Lamanite lines.
This combination of slaughter and escape did free Gid and his men from the necessity of watching those men, and that freedom allowed them to march faster and fight more freely when they did return to the assistance of Helaman’s army in the field. Even though the loss of the prisoners was not ideal, the end result of that loss was the preservation of a Nephite army, and a valuable exchange.