The Lamanites’ eventual redemption will result from their repentance, but the Ammonihahites will be destroyed because of their refusal to repent. Alma clearly sets up repentance as the hinge upon which the people’s fate turns. It is not Yahweh’s favoritism. Indeed, every individual must repent of his or her sins and come unto Yahweh with a broken heart. For the whole world, the possibility of destruction is symbolic. For the Ammonihahites it was physical and imminent.
Alma’s admonition to the Ammonihahites has an added subtext, however, and that is the idea that the Lord has higher expectations of those to whom he has revealed his gospel (in this case, the Nephite religion which Ammonihah is rejecting). The scriptures repeat this theme: “For unto whomsoever much is given, of him shall be much required” (Luke 12:48; see also D&C 82:3).