Among the Nephites, the Gadiantons become the major political and social “party.” They have the rule, and they have the hearts of the majority of the people. What this means is that they have control over the resources that will allow them to increase their wealth. Note that the result of their rule is “murders, and their plunderings, and their stealings.”
Each of these terms is semantically loaded. They are all bad things. However, changing the context away from Mormon’s perspective and towards that of the Gadiantons and the people, the terms might be recast as military conquests – with the “murders” becoming death in wars of conquest, and the “plunderings and stealings” being the tribute assessed upon the conquered land. As we saw with the Anti-Nephi-Lehies, there is also the possibility that “murders” in Mormon’s vocabulary also included ritual human sacrifice.
We certainly cannot be certain that this is what Mormon means in this case, but since it was an aspect of the cult of war that otherwise fits this description of the method of accumulation of wealth, we must at least consider the possibility that such a heinous practice was being instituted among the Nephites as part of their apostasy from traditional Nephite religion and their adoption of the Lamanite social-religious order.