Historical: We do not have data for the nature of early warfare. Our best information comes from Aztec times, which post date the Book of Mormon by some six hundred years. We do know that from the times of the Classic Maya through the Aztec domination, there was an increase in the cult of sacrifice, which reached its apogee in the Aztecs. Thus the Aztec penchant of taking as many prisoners as possible may not be the model that is applicable to earlier times. We have seen that the Maya preferred captives of high status, and we see in this verse the capture and retention of the chief captains. These prisoners were sufficiently important that they were returned to the land of Nephi (the homeland of the Lamanites).
The rest of the prisoners were killed. We do not know how they were killed, but it is possible that in the Mesoamerican context in which we are placing the Book of Mormon that they were sacrificed. Even in Aztec times, during a long campaign where they were not returning home, captives would all be sacrificed in the field (Ross Hassig. Aztec Warfare. University of Oklahoma Press, 1988, pp. 115-6). It is possible that this is the reason for the slaughter of the non-elite captives. Not only would this be a religious act of human sacrifice, but one that was pragmatic in that the attacking army did not have to deal with the care and guarding of so many prisoners.
The situation differs from that which we saw on the eastern front where the Lamanites were harboring captives. In that case, the Lamanite armies were bogged down and attempting to hold on to captured cities. In such a case, the captives could be kept for ransom because the army was not on the attack. The western front was more dynamic, and apparently the leaders in charge wanted to keep their ability to move rapidly intact.