Moroni pronounces a judgment that seems harsh, but which was actually quite merciful. What he had was a group of people who were traitors, and who had willingly left the Nephite political protection and were attempting to join with the sworn enemies of the Nephite people. In that traitorous condition, all of the people merited the death penalty. Rather than execute that penalty, Moroni gave the people the opportunity to “enter into a covenant to support the cause of freedom.” Those who did enter in to the covenant were brought back into Nephite society with no penalty. Those who refused remained in their traitorous state and were subject to the death penalty.
As at other times in the Book of Mormon we see the power of the oath in these people. The power of the personal covenant was strong enough that Moroni trusted those who accepted the covenant. The oath was so strong that those who could not enter in to the covenant chose death instead. These ancient oaths bound the people better than all of our signed contracts do today.