Taken as a whole, the history of the Book of Mormon shows that the Nephites were more righteous than the Lamanites. However, the Lord eventually destroys the Nephites for their rebellion. This is justified because they had sinned against the light of the gospel. For of him unto whom much is given much is required; and he who sins against the greater light shall receive the greater condemnation (DC 82:3). The Nephites knew the truth and had openly rebelled against it, Now they did not sin ignorantly, for they knew the will of God concerning them, for it had been taught unto them; therefore they did willfully rebel against God (3 Nephi 6:18).
The Lamanites, on the other hand, had become wicked because of the wicked traditions of their fathers. Laman and Lemuel must be held partly responsible for the wickedness of their descendants. Speaking to the children of Laman, Lehi said, if ye are cursed, behold, I leave my blessing upon you, that the cursing may be taken from you and be answered upon the heads of your parents. Wherefore, because of my blessing the Lord God will not suffer that ye shall perish; wherefore, he will be merciful unto you and unto your seed forever (2 Nephi 4:6-7). This promise was repeated to the children of Lemuel, demonstrating that the Lord acknowledged that the Lamanite filthiness came because of their fathers.