The days of final judgement are days of justice, but the days of justice do not necessarily bode destruction for all. It is a time of destruction of the wicked, but not of the righteous. Nephi specifically notes that the righteous will include those who repent. Thus the final reward is not for those who can avoid all sin, but also for those who can recognize their sinful state and repent in that last day.
This salvation of the repentant does not mean that Nephi is promoting death bed repentance but rather only emphasizing the ability of repentance to create a reconciliation with the Lord sufficient to be found among those who will survive the destructions of the wicked in the last days.