Why were the Lamanites now fighting amongst themselves? They ought to have been dancing in the streets. They had finally gotten rid of the pesky Nephites. The world should have been wonderful for them, but what was happening?
Is this part of human nature? Has this ever happened in history before? The Lamanites were probably not a single group of people with one king. There were many tribes, and their world had deteriorated. When everything collapsed at the end of 4 Nephi, Mormon makes a point of noting that the people became tribalized, regionalized, and divided. There had been great unity and there were no manner of ites, but then corruption entered. Everyone split off into their own social, economic, or ideological groups.
Many of these groups had a common enemy—the Nephites. They were ready to get rid of them. However, once they had, we see the occurrence of a familiar pattern. If a political group is only brought together by an enemy, once that enemy is gone, there is not much to hold them together any longer. They begin dividing up the spoils and wondering who would get what. They inevitably turn on each other and start fighting. If a civilization is not a peaceful one to begin with, they are likely not going to become peaceful when they achieve power and success.