It will be recalled that in 78 B.C.Mormon, the historian, decided to suspend the record of Alma for a few chapters and do a back flash so the Book of Mormon would incorporate the records of the missionaries who had gone down to teach the gospel to the Lamanites. Now we are about to do a back flash within the original back flash so we will know what happened to Aaron after he separated from Ammon.
Mormon wants us to realize that when the sons of Mosiah went down to preach to the Lamanites they discovered themselves in a most decadent and apostate people. Among the most apostate were former Nephites. Two of them are named specifically in verse 3 as the Amalekites and Amulonites. The Amalekites do not have any background history in the Book of Mormon, we just know that they were once Nephites and they apostatized and joined the Lamanites.
But we do know a lot about the Amulonites. They are the descendants of Amulon, who was one of the leaders of the wicked priests of king Noah. He and his associates had kidnaped a number of Lamanite girls for their wives. Earlier they had joined with king Noah in burning at the stake the prophet Abinadi. These were truly decadent and wicked people.