Alma’s argument here is unique in the scriptures. Alma suggests that repentance requires “a punishment.” What he is suggesting is that the entire plan of life follows Lehi’s law of opposition, and that even the plan itself must be described in terms of balanced opposites. What Alma is suggesting isn’t that we must be punished, but rather than there must be an eternal consequence of our actions that would lead to the opposite effect of the promised reward of the plan of happiness, “which was as eternal as the life of the soul.”
This “punishment” that Alma places opposite the plan of happiness also has its foundational understanding in the teachings of Lehi:
2 Nephi 9:8-9
8 O the wisdom of God, his mercy and grace! For behold, if the flesh should rise no more our spirits must become subject to that angel who fell from before the presence of the Eternal God, and became the devil, to rise no more.
9 And our spirits must have become like unto him, and we become devils, angels to a devil, to be shut out from the presence of our God, and to remain with the father of lies, in misery, like unto himself; yea, to that being who beguiled our first parents, who transformeth himself nigh unto an angel of light, and stirreth up the children of men unto secret combinations of murder and all manner of secret works of darkness.
Lehi taught that if there were no atonement then our souls were destined to become “devils, angels to a devil.” This is the opposite pole of the plan of happiness, which logically has us become “gods, angels to a god” even though Alma does not state it in precisely this way.