The final judgment will bring justice and, hence, the destruction of the wicked. However, the righteous—among whom Nephi includes those who repent—will be spared. Thus, the final reward is not for those who avoid all sin (a mortal impossibility), but for those who recognize their sinful state and repent.
This salvation of the repentant does not mean that Nephi is promoting deathbed repentance. Rather he is emphasizing how repentance can create reconciliation with Yahweh so that the repentant will survive in the last days when the wicked are destroyed.