Nephi comes to the issue of inequality. They have desired to “get gold and silver.” That in and of itself is not the problem. Rather it is that “ye have set your hearts upon the riches and the vain things of this world.” They desire them so much, that they are willing to reject their religious and political heritage to acquire them. When Mormon suggests that they are willing to “do murder, and plunder, and steal, and bear false witness against [their] neighbor,” he is listing traits that have, in other occasions, been associated with the Gadianton robbers. Nephi is telling them that they have allowed the Gadianton’s to take over and change Nephite religion and social structures because they have apparently brought wealth and prosperity with them. Reading between the lines, this is the result of the trading networks that are being set up.
Nephi also knows that this is a false prosperity, and that it will disappear when the Lord withdraws His protection. That is, in fact, exactly what will happen in the not too distant future.