Capping this wicked century is the reappearance of the Gadianton robbers. Like other events from Nephi3’s record, this appearance is symbolic as much as it is historical. These Gadiantons show up because that is what Gadiantons do. They cause the destruction of a people. They did it before, and they are beginning to do it again. (See Helaman, Part 1: Context, Chapter 3, “The Gadianton Robbers in Mormon’s Theological History: Their Structural Role and Plausible Identification.”)
The idea that they “did traffic in all manner of traffic” describes widespread trade. Trade has always been part of the Nephite world, but it has also been the most probable source of foreign and apostate values. (See commentary accompanying Jacob 2:12–13.) The righteous Nephites could not have become “rich” without trade (v. 7), but Mormon mentions it now because of its role in Nephite social ills. (See Alma, Part 1: Context, Chapter 1, “Historical Background of the Book of Alma.”)