This is the story that Mormon was setting up in order to contrast it with the time of peace and prosperity. That internal peace is disrupted, and not simply by descendants of Zarahemla, but more importantly by the sons of the king and the high priest. The most important men in the society, with the exception only of their fathers, these sons had tremendous influence. They used their influence to lead many away from righteous principles and away from the covenants of the church. Note the final line of verse 9: “giving a chance for the enemy of God to exercise his power over them.” This is the negative aspect of the promise of the land. They were prosperous if they were righteous. Now that they were being drawn away from righteousness, the door was opened for a cursing upon the people.
How was it that such important sons might rebel? Had they not been taught by the best of fathers and mothers? Mormon is telling us that there were other factors at play. When the Nephites became prosperous, one of the unstated reasons and results was that there were more connections made with cities outside of the Zarahemla hegemony. Those cities were not believers, but were probably representative of at least as much prosperity as the Nephites, and perhaps they were even more prosperous than them. In other words, living in a tempting world affected these sons. As prominent sons, they were in a position to benefit from the external connections, and perhaps were affected by them more strongly than others for that reason.
Parents have a responsibility to teach. Parents may honestly do their best to fulfill that responsibility. Nevertheless, agency allows children access to the ideas and temptations of the world, and can make that choice themselves, in spite of the earnest desires of their parents. The story of the rebellion of the sons of Mosiah and Alma the younger will have a happy reconciliation with their father’s hopes and teachings, but other equally faithful parents will not see that end. For that reason, hope does not end with this life.