Thirteen years prior, Mormon had given up on his wicked armies. They had become proud and boastful, going on the offensive for the sake of revenge (Mormon 3:9-11). Now, the situation is different. The Nephites cannot go on the offensive. Mormon knows that his armies will remain on the defensive until they are ultimately destroyed. Undoubtedly, his change of heart was because of the greatness of his heart. He was full of compassion, even for his wicked people, and the greatness of his soul would allow him to stand as an idle witness no longer.
Hugh Nibley
"In this crucible of wickedness the true greatness of Mormon shines like a star as he calls his son to action, telling him that no matter how bad things are, we must never stop trying to do what we can to improve matters, 'for if we should cease to labor, we should be brought under condemnation; for we have a labor to perform whilst in this tabernacle of clay' (Moroni 9:6). In this spirit Mormon took over command of the army even when he knew that all was lost, 'for they looked upon me as though I could deliver them from their afflictions. But behold, I was without hope' (Mormon 5:1-2). His is the predicament of the true tragic hero: 'I had led them, notwithstanding their wickedness, . . . and had loved them . . . with all my heart; and my soul had been poured out in prayer unto my God all the day long for them; nevertheless, it was without faith, because of the hardness of their hearts' (Mormon 3:12)." (Since Cumorah, p. 400)