As a witness of the righteousness of the Lamanites Mormon shows them destroying the Gadiantons from among them. This will again assure their continuation, as it is the Gadiantons who will be behind the dissolution of the Nephite polity. On a more historical level, this probably indicates that the Gadiantons were considered foreign to the Lamanites, and therefore did not have the same base of support in the clans of the Lamanites. They were therefore easier to find and isolate without that public support base.