Even though the people accept Nephi as a prophet, the Gadiantons need to discredit their belief in him as a prophet, particularly at this point in time where the gathered crowd appears at least somewhat sympathetic to Nephi. The easiest way to do that is to deal with Nephi’s prophetic threat to the Nephite nation.
Why were the Gadiantons so certain that they were powerful and the cities were great? Didn’t the Nephites only recently lose most of their lands and were forced to retreat into Bountiful? Yes, but since that point there was a great economic boom, and they were richer than they had been before. In times of boom, optimism is a much more common coin of public opinion. The increase in economic position created a presumption that there was a concomitant increase in political and military power.
What the Gadiantons do not realize is that the very economic forces that were allowing them to become wealthier would lead to their downfall. The desire for wealth was pushing the Nephites into wars for tribute – the Gadianton “stealings and plunderings.” This inter-city warfare and domination would have a region-wide repercussion as alliances shifted, and dominated cities would attempt to realign themselves with more powerful city-states to remove the burden of tribute – or even better, reverse the direction. The Mesoamerican spiral of warfare is beginning in earnest, and will yet turn on these prosperous Gadiantons.