In Mormon 4, Mormon said that at that time the Nephites were being swept off as “a dew before the sun.” One may picture the ancient weapons such as the macuahuitl weapon and visualize it sweeping away the losers. How graphic that is. Many were on the run or involved in mass desertions, and women and children were captured and abused.
In Moroni 9:24, Mormon ended his letter by saying likewise, “Many of our brethren have deserted over to the Lamanites.” One may ask why they would have done that. They likely did not want to die, but how could they have avoided being killed? They would have been required to “deny the Christ” (as we learn from Moroni 1:2–3). This was more than a political war. The people that chose to stay with Mormon did so at least partly for religious reasons, and some chose to die rather than default on their faith.