In this verse, Alma homes in on the most important part of his sermon. The dilemma of agency is that choice can lead to eternal condemnation, yet damnation is not the preferred outcome of earth life. The way out of temporal and spiritual death is through the Son, the Atoning Messiah—whom the Nehorites deny. The Son’s atonement allows repentance, which removes the effects of sin while retaining the benefits of agency. Human beings may learn to choose Yahweh, not harden their hearts, remove their sins by repentance, and then stand justified before the bar of God. Mercy may override justice because the atonement has removed the contamination of their sins.
From a theological standpoint, justice will always prevail. Justice requires that a sinner be excluded from Yahweh’s presence. Mercy does not contradict this requirement. What mercy does is remove the sin, therefore making justice inapplicable.