Alma 54:2 Moroni felt to rejoice exceedingly at this request
Moroni had previously defeated Lamanite armies and then sent them home if they would take an oath never to fight anymore (Alma 44:20). However, after many Nephites had been taken prisoner, he changed his policy, ordering Teancum that he should retain all the prisoners who fell into his hands...as a ransom for those whom the Lamanites had taken (Alma 52:8). Having successfully taken many Lamanite prisoners in the battle for the city of Mulek (Alma 52:39), Ammoron's request was just what Moroni wanted. The Nephites had been supporting the Lamanite prisoners for almost a year, and it had become a burden. Interestingly, the orderly exchange of prisoners never takes place.
Alma 54:11 it supposeth me that thou art a child of hell
Moroni was a passionate man. Elder Vaughn J. Featherstone stated, "In a letter to Ammoron...we catch more of the spirit of this great man. As President Lee would have said, he wasn't a shrinking violet. There was no misunderstanding his message. It wasn't watered down or diluted. " (Ensign, Nov. 1975, p. 7) He was so angry with Ammoron that he started his letter with a verbal whipping, calling him among other things, "a child of hell." But Moroni is unwilling to make a deal with the devil or his children unless it is to his sole advantage. Accordingly, he makes Ammoron a lopsided offer.
Alma 54:17 your fathers did wrong their brethren...they did rob them of their right to the government
There are several aspects of Ammoron's letter which must have angered Moroni. Ammoron's use of traditional Lamanite rhetoric about the right to rule must have been particularly infuriating. What did Ammoron care about Lamanite claims to governmental authority? He was a descendant of Zoram (v. 23). It wasn't his ancestors who were "wronged." He and his brother Amalickiah had become Lamanites not to preserve these ancient claims but to stake their own claims over the Nephites. Moroni, of course, recognized this and became even more angry, because he knew that Ammoron had a perfect knowledge of his fraud (Alma 55:1).
"Ammoron referred, of course, to Laman's complaint that Nephi 'thinks to rule over us,' when Laman himself claimed the right of rulership. 'We will not have him to be our ruler; for it belongs unto us, who are the elder brethren, to rule over this people' (2 Nephi 5:3). Ammoron represents the war as a continuation of an ancient feud between the two sets of brothers in Lehi's family. That hardly makes sense to us. Would countless thousands of men hundreds of years later throw themselves into battle simply to reclaim an ancient right? It is all the more puzzling because after the landing in America, Nephi and his descendants made no claims that we know of to rule the Lamanites. Quite to the contrary, Nephi withdrew from the site of the first landing by command of the Lord, leaving the area to his brothers (2 Nephi 5:5- 7). The first King Mosiah also withdrew by command of the Lord (cf. Omni 1:12-13), pulling back from the Lamanites and not forcing his rule on them. Until near the end, the Nephites never fought aggressive wars. The Lamanites were the ones to attack, not the Nephites." (John M. Lundquist and Stephen D. Ricks, eds., By Study and Also by Faith: Essays in Honor of Hugh W. Nibley on the Occasion of His Eightieth Birthday, 27 March 1990, 2: 54)
Alma 54:20 I will grant to exchange prisoners according to your request, gladly
Ammoron is glad to exchange prisoners even if it means one Nephite family for one Lamanite soldier. Hereby, Moroni seems to have pulled off quite a diplomatic victory. But Moroni's passions take over. He becomes even angrier; he cannot stand the thought of doing anything which helps Ammoron in the least. Accordingly, he changes his mind about the deal and plots to regain the prisoners by stratagem (see Alma 55:1-2).