As should have been understood, Mormon expressly declares that the Gadiantons are behind the murders. The increase in wickedness has come directly on the heels of the “riches of the world.” Mormon spent time noting the increase of trade, specifically with the north. Now, the Gadiantons are here. The connections between the Gadiantons and the north will continue to be made through Mormon’s coming text.
As the Gadiantons had done before, they achieve their goals through secret murder rather than through the voice of the people. Thus, they move to make a change in the Nephite government by murdering Cezoram and his son. That removed two sitting Chief Judges, and likely threw the question of succession open to their own desires.