Moroni’s letter to Ammaron is a really interesting one to read. It’s hard to read it without wondering what Moroni was attempting to gain by sending such an angry and heated letter. Certainly, telling someone that their brother who had just been killed was in hell was not exactly likely to inspire reconciliation. And telling someone that you are planning to march into their land and turn the war into a war for extermination is unlikely to pacify a foe.
So why does Moroni do it? It seems to me that there are two options. One is that he simply lost his temper and was angry. This would not be out of character for Moroni. It would also explain why Teancum later kills Ammaron in a rage and is ultimately killed. Perhaps the people expected that killing Amalickiah would end the warfare and they were disappointed that the war continued. These are all possibilities.
Another option is that Moroni is deliberately trying to goad Ammaron into overreaction. Every time the Lamanites have overreached it has ultimately benefited the Nephites. In their blind aggression, the Lamanite forces were prone to slip up and make mistakes. It is possible that Moroni deliberately sought to anger the Lamanities in order to gain an advantage. I also find this interpretation plausible and consistent with Moroni’s overall tactics.