Nephi makes two categories of sins on the people. The first is the acceptance of the Gadiantons, and the second is the “pride which ye have suffered to enter your hearts.” This pride led to the use of wealth as a means of social distinctions and the establishment of economic hierarchies. Note that Nephi says that they have been “lifted.. up beyond that which is good.” This “lifting up” is part of the process of creating the social distinctions.
The acceptance of the Gadiantons is interesting. Nephi notes that the Nephites have united themselves to this “secret band.” If the majority of people have united themselves to it, it is hardly a “secret band” in the connotation that they presence is secret. The secrets are in the combinations, not in the existence. Nephi notes that this is something that has come among them, and therefore Nephi is declaring it to be foreign in origin – foreign at least in the idea that it does not belong in traditional Nephite society. It is an idea from the outside, fomented by people on the inside.