The Amalekites were Nephite dissenters living in Lamanite territory, most of them followers of the order of Nehor. With the Lamanites and the people of Amulon they built the city of Jerusalem, near the borders of Mormon, around 90 BC (Alma 21:2). They worshipped in synagogues built after the order of the Nehors and held that God would save all men (Alma 21:4, Alma 21:6).
The Amalekites and Amulonites were harder in their wickedness than the Lamanites themselves, and they pushed the Lamanites to harden their hearts further (Alma 21:3). When Aaron, a son of King Mosiah, preached to them at Jerusalem, an Amalekite contended with him, denying that the Son of God would come to redeem mankind (Alma 21:6-8). Of all the Amalekites only one was converted (Alma 23:14).
The Amalekites and Amulonites stirred up the unconverted Lamanites against the Anti-Nephi-Lehies, their converted brethren (Alma 24:1), and made up the greatest number of those who slew them (Alma 24:28). After that slaughter they remained angry at their loss and again sought to destroy the people of Anti-Nephi-Lehi, prompting Ammon to lead that people away to Nephite lands (Alma 27:2, Alma 27:12).
In the war that opened in the eighteenth year of the judges, Zerahemnah made chief captains over the Lamanite army, all of them Amalekites and Zoramites, because the Amalekites were of “a more wicked and murderous disposition” than the Lamanites (Alma 43:6). These Amalekites and Zoramites led the Lamanites in battle against the Nephites (Alma 43:44). Unlike the rest of the army, who fought nearly naked, the Zoramites and Amalekites were armored (Alma 43:20). The record notes that a people once enlightened by the Spirit of God who then fall into sin become more hardened, so that their state is worse than if they had never known these things (Alma 24:30).