Nephi classifies the people’s sins in two categories: accepting the Gadiantons and becoming proud. Pridefulness led to the creation of social classes and economic hierarchies. When Nephi says that they have been “lifted… up beyond that which is good,” he is describing their acceptance of social distinctions.
Nephi characterizes the Gadiantons as a “secret band” to which the people have “united” themselves. In that case, it cannot be a “secret band,” even though its “combinations” and oaths are. By saying it “has come among you,” Nephi declares it to be foreign, not part of the Nephite tradition, an idea from the outside embraced by insiders who should have known better.