The contrast between the Lamanite and Nephite response to the presence of the Gadiantons is significant. Mormon is foreshadowing a time when the Nephites would be so much more wicked than the Lamanites that the Nephites would be destroyed while the Lamanites survived. The conditions will be a bit different in the end, because it will be a combination of Lamanites and Gadiantons that will destroy the Nephite nations (which Mormon foretold in Helaman 2:13).
Verse 21 reprises the nature of these Gadiantons. They have secret oaths to protect one another. They desire to gain power and prestige through “their murders, and their plunderings, and their stealings.” They are out to undermine other societies and peoples for their own gain.