Amulon and his fellow priests had kidnapped then married Lamanite women. Their collective descendants became known as the Amulonites. They did join the Lamanites (v. 35) and become one of the most wicked groups in the history of the Book of Mormon. George Reynolds condenses their history:
"They were Nephites on their father's side and Lamanites on their mother's but by association and education were of the latter race. Many of them, however, were displeased with the conduct of their fathers and took upon them the name of Nephites and were considered among that people ever after. Of those who remained Amulonites many became followers of Nehor and were scattered in the lands of Amulon, Helam and Jerusalem all of which appear to have been limited districts in the same region of country. In the later years the sons of Mosiah and their fellow-missionaries preached to them but not one repented and received the gospel message; on the contrary, they became leaders in the persecutions carried on against the suffering people of Anti-Nephi-Lehi and were those who, with the Amalekites, slew the greater number of that unoffending people who suffered martyrdom. In the succeeding war with the Nephites (B.C. 81), when Ammonihah was destroyed, nearly all the Amulonites were killed in the battle in which Zoram, the Nephite general, defeated the Lamanites. The remainder of the Amulonites fled into the east wilderness where they usurped power over the people of Laman and in their bitter hatred to the truth caused many of the latter to be burned to death because of their belief in the gospel. These outrages aroused the Lamanites and they in turn began to hunt the Amulonites and to put them to death. This was in fulfillment of the words of Abinadi, who, as he suffered martyrdom by fire at the hands of Amulon and his associates, told them, 'What ye shall do unto me, shall be a type of things to come' by which he meant that many should suffer death by fire as he had suffered." (A Dictionary of the Book of Mormon, pp. 59-60)