Redaction: This is Moroni’s recounting of the events. Moroni had seen the destruction of his people, and he certainly understood his father’s contention that it was the Gadianton robbers who had led to the destruction of the Nephites, both before the coming of the Savior to Bountiful as well as the final destruction. Moroni is therefore using the term “robbers” in the sense of the representatives of the secret combinations.
These are not brigands, these are the precursors of the named Gadianton robbers of the end of Nephite society. Moroni is tying together the destruction of the Jaredites with the destruction of the Nephites, and placing the blame for both on secret combinations, or the “robbers.”