Moroni charges Pahoran with transgressing Yahweh’s laws which require him to sustain the egalitarian ideal and, therefore, to provision his armies. By ignoring the Nephite ideals, he allows, even facilitates, a Lamanite take-over. Moroni continues to hammer away at his main point: withholding supplies and reinforcements is simply a symptom of a much greater sin—that of accepting the Lamanite worldview.
He also once again threatens to depose and possibly execute Pahoran and his supporters, this time claiming a revelation as his authority. He has not mentioned this revelation before. While this might suggest that he received the revelation during the dictation of the letter, I suspect that his angry state was anathema to such spiritual communication. I believe that this was a principle that had been revealed to him and upon which he made his decisions, including the decision to send this quite vitriolic letter.