Even though Nephi had spoken out against the Gadianton-led government, and had provided rather dramatic proof that he could knowledgeably tell the future, the majority of the people did not believe his prophecy that they would be destroyed except they should repent. Thus, they continued doing as they had done.
As Mormon continues to develop this story, he continues to build his case that the Gadiantons were the harbingers of destruction. He does not overtly name them in verse 2, but there is no doubt that “this secret band of robbers who did carry on this work of destruction” were the Gadiantons.