Alma 25:1 - 25:5
Alma 25:1 And behold, now it came to pass that those Lamanites were more angry because they had slain their brethren; therefore they swore vengeance upon the Nephites; and they did no more attempt to slay the people of Anti-Nephi-Lehi at that time.
Now the Lamanites, who had slain some of the Anti-Nephi-Lehis, became angry at the Nephites, accusing them of causing this condition of converted Lamanites to existed in their land and decided to take it out on the Nephites. They did not kill anymore Anti-Nephi-Lehis.
I guess the Lamanites were not as blood thirsty as we might have thought, because of what we have read so far in the Book of Mormon. They appear to have consciences and the killing of people who put up no resistance, at least at the time, was repugnant to them as a whole. There might have been a few who would have killed all of the people of God but were constrained by the majority.
Alma 25:2 But they took their armies and went over into the borders of the land of Zarahemla, and fell upon the people who were in the land of Ammonihah and destroyed them.
The Lamanite army crossed over into Nephite territory and took out their anger by attacking the city of Ammonihah and completely destroyed it.
Alma 25:3 And after that, they had many battles with the Nephites, in the which they were driven and slain.
Mormon tells us that this was the beginning of many battles which will be described in the future between the Nephites and Lamanites.
Alma 25:4 And among the Lamanites who were slain were almost all the seed of Amulon and his brethren, who were the priests of Noah, and they were slain by the hands of the Nephites;
The result of these confrontations was death of nearly all of the descendants of Amulon and his associates who were the priest of Noah.
The time of Noah was about 60 or so years previously.
Alma 25:5 And the remainder, having fled into the east wilderness, and having usurped the power and authority over the Lamanites, caused that many of the Lamanites should perish by fire because of their belief—
When the Lamanite army was driven into the east wilderness by the Nephites, the Lamanite leadership under the Amulonites, began to cause the Lamanites were not so sure about their cause, were put to death by fire.