Captain Moroni was very angry because the Lamanites were a present and powerful threat to his people, but the king-men were refusing to help. He was stern. But what justified his severe treatment of the king-men? The Nephites had their backs against the wall. If they did not use every possible resource at their disposal, Bountiful would be taken and they would be encircled, their lands invaded, and it would all be over.
It is important to notice that Moroni obtained authority to impose martial law here. He went to the people and got authority as commander-in-chief of the military to be able to deal with the king-men. Alma 51:15 tells us that, “He sent a petition, with the voice of the people, unto the governor of the land, desiring that he should read it, and give him [Moroni] power to compel those dissenters to defend their country or to put them to death.” He gave the king-men an opportunity to take an oath. If they would not fight to defend the country, which was a normal legal obligation of all able-bodied men, they could then be put to death for not fighting. He could require them to swear the oath of allegiance and could put them to death for not swearing the oath. He put at least some of them in jail, and they sat there for six years until this war was over. The situation was urgent, so he planned to take care of the dissenters, but not immediately.
Book of Mormon Central, “Why Did Moroni Speak of Pulling Down Pride? (Alma 51:17),” KnoWhy 430, (May 3, 2018).