Ammoron had countered each of Moroni’s accusations, but of course he did so with his own justifications, and clearly those reasons were not justified in Moroni’s eyes. Even though Moroni had achieved the basic intent of his letter (to have a prisoner exchange at three to one), he is nevertheless angry at Ammoron’s distortion of the causes of the events. Moroni knows that Ammoron has the true knowledge of the nature of the Nephite and Lamanite relations because Ammoron was Nephite until recently. Moroni sees the blatantly Lamanite reasoning in Ammoron’s epistle as a “fraud.” It is a new justification of the world that Ammoron has conveniently adopted.
One of Ammoron’s justifications for war was the death of Amalickiah. Of course that death occurred during a war that was already being waged, so that could not be seen as a justification for the original attack.