Because of the chronology of the Book of Mormon can be confusing, it is hard to remember what events had led up to this vicious attack on Ammonihah. The background of the story is told in Alma 24. The people of Anti-Nephi-Lehi were Lamanites who had made a covenant that they would no longer participate in wars or shed any man's blood (Alma 24:17). Incited by the Nephite dissident leaders among them (who ironically were also of the order of the Nehors), a Lamanite army attacked the people of Anti-Nephi-Lehi who had prostrated themselves before them to the earth, and began to call on the name of the Lord (Alma 24:21). The Lamanite army killed 1005 helpless, unarmed believers. This had a remarkable effect on them. They were disgusted that they had taken the lives of their fellow Lamanites and vowed from that time that they would take vengeance upon the Nephites (Alma 25:1). This vengeance was fully exacted upon the people of Ammonihah (Alma 25:2).
It should be evident from the preceding chapters that the Ammonihahites deserved the punishment they received. In case you've forgotten, let's review what they had done to deserve this fate:
In spite of all this wickedness, they retained an attitude of rebellion which is best encapsulated in their defiance of Alma when they said, We will not believe thy words if thou shouldst prophesy that this great city should be destroyed in one day (Alma 9:4). But behold, in one day it was left desolate; and the carcases were mangled by dogs and the wild beasts (v. 10).