Mormon tells the story in a way that depicts the Lamanites as duplicitous. Actual history might have been a little different. Mormon also indicates that Alma had surrendered his people to the Lamanites, and the resulting Lamanite domination was typical of the tribute system that was typically established. Alma’s people did indeed retain their lives, but they entered into a tribute relationship.
The backstory of the priests of Noah comes to the fore when Amulon, one of those priests, is given the authority of the Lamanite king over Alma’s people.