The spiritual instability of the Nephites is reflected in the fact that the whole Church split in two very shortly after Alma disappeared. Helaman and his two brothers tried to rally the people in righteousness and the resentment of those who rejected their message immediately boiled over into open rebellion. And it was not merely because the rebels wanted to reject the message. They wanted to murder the messengers. Alma's three sons were doing missionary work at the risk of their lives.
Satan never has trouble finding someone who has secret ambitions and is willing to launch a civil war to gain power. Amalickiah turned out to be Satan's newest recruit and he wanted to be king.