When the Nephites “began to boast in their own strength, and began to swear before the heavens that they would avenge themselves of the blood of their brethren who had been slain by their enemies” (Mormon 3:9), Mormon refused to command them anymore. Like Ether of an earlier day (see Ether 13:14, 15:33), Mormon began to “stand as an idle witness to manifest unto the world the things which [he] saw and heard” (Mormon 3:16). He had led the Nephites and had loved them, and had prayed earnestly for them “all the day long” (v. 12), but he refused to follow them into even greater wickedness.
Self-defense and vengeance are not the same. The Lord sometimes justifies his people in fighting to defend their homes and families from attack, but he does not justify offensive war. The Lord said, “Vengeance is mine, and I will repay” (v. 15). It is God who deals out retribution unto men. In taking the offensive, the Nephites went off to battle without the sanction of the Lord (see Mormon 4:4), which resulted in the eventual destruction of an entire nation.
Though Mormon couldn’t teach his people because of the hardness of their hearts, he tried to teach future readers to learn from history and avoid the terrible mistakes his people had made (see Mormon 3:17–22).