By this time in the Book of Mormon, the people are not divided on racial lines as much as religious and political lines. There is a great amount of mixing of Nephite and Lamanite blood. The Amulonites are the descendants of the priests of Noah who were half Nephite (fathers) and half Lamanite (mothers). The Amalekites are apostate Nephites. The story of their dissent from the Nephites is not given by Mormon. They are a different group than the Amlicites spoken of in Alma 2-3. Given that these Amulonites and Amalekites practiced religion after the order of the Nehors (a recent religious movement), the dissent of the Amalekites probably occurred not long before Aaron showed up (see Alma 1:15-16).
The story of these two groups is crucial to the book of Alma. It won't be long before we will be reading about the many, hard-fought wars between the Lamanites and Nephites. It is the Amulonites and Amalekites who are the instigators and main military leaders of the Lamanite armies. They become a perpetual thorn in the side of the Nephites. Of their wickedness, Mormon commented, And thus we can plainly discern, that after a people have been once enlightened by the Spirit of God, and have had great knowledge of things pertaining to righteousness, and then have fallen away into sin and transgression, they become more hardened, and thus their state becomes worse than though they had never known these things (Alma 24:30).
"Both of these groups of people believed in the 'order of the Nehors', and they were so hardened in wickedness that only one Amalekite and no Amulonites were converted by the four sons of Mosiah and their companions (Alma 23:14)." (Daniel Ludlow, A Companion to Your Study of the Book of Mormon, p.208)
"When the Prophet [Joseph Smith] had ended telling how he had been treated [by apostates], Brother Behunnin remarked; 'If I should leave this Church I would not do as those men have done: I would go to some remote place where Mormonism had never been heard of, settle down, and no one would ever learn that I knew anything about it.' The great Seer immediately replied: 'Brother Behunnin, you don't know what you would do. No doubt these men once thought as you do. Before you joined this Church you stood on neutral ground. When the gospel was preached good and evil were set before you. You could choose either or neither. There were two opposite masters inviting you to serve them. When you joined this Church you enlisted to serve God. When you did that you left the neutral ground, and you never can get back on to it. Should you forsake the master you enlisted to serve, it will be by the instigation of the evil one, and you will follow his dictation and be his servant.' He [further] emphasized the fact that a man or woman who had not taken sides either with Christ or Belial could maintain a neutral position, but when they enlisted under either the one or the other they left the neutral ground forever." (Juvenile Instructor, Aug. 15, 1892, p. 492 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 228)