Moroni’s judgment seems harsh but is actually quite merciful. The captured Amalickiahites were traitors who had willingly left Nephite political protection to join with the Nephites’ sworn enemies. Treason has always been a capital offense; but rather than execute them, Moroni offered the people the option of covenanting “to support the cause of freedom.” Those who accepted this option returned to Nephite society with no penalty. Those who refused remained subject to the death penalty.
As at other times in the Book of Mormon, we see the power of the oath for these people. (See commentary accompanying 1 Nephi 4:31–35 and Alma 44:5–20.) It was so strong that Moroni trusted those who accepted it, and those who could not enter into the covenant chose death instead.