This verse is Mormon’s explanation of events to follow. Since his intention is to describe preaching the gospel to the Lamanites, Mormon explains why Aaron received a very different reception than Ammon did.
We already know that the antipathy between the Lamanites and Nephites could be quite strong, as the reaction of Lamoni’s father to Ammon shows (Alma 20:13). However, Ammon’s stay in Lamoni’s court, even before the conversion experience, shows a much less volatile relationship. No one reacted with strong hatred, although the original welcome was not exactly a complete embrace. Perhaps the old antagonisms may have been diminishing, with most Lamanites casting the Nephites as an opposing political entity, but not as objects of extreme hatred. This changing attitude suggests one reason for the success of the Lamanite mission.
However, Mormon reminds us of the Amalekites and the Amulonites, both of recent Nephite heritage but living among the Lamanites. For them, the wounds of separation still smarted, and their hatred of the people upon whom they had both turned their backs and previously engaged in battle must have been extreme. Mormon understood the phenomenon of hostility experienced by apostates toward the group from which they have apostatized. Thus, he suggests, these two former Nephite peoples have escalated in intensity whatever antipathy the Lamanites felt for the Nephites.