This story has a moral that is so important that Moroni clarifies it by making it explicit. He inserts his own words into the text. Why is the story so important? In verse 21, he tells us that their oaths and combinations “have caused the destruction of his people of whom I am now speaking, and also the destruction of the people of Nephi.”
Moroni therefore reasonably cautions his future readers to make sure that they do not fall to the same oaths and combinations. Ultimately, nations who embrace those elements self-destruct, “they shall be destroyed; for the Lord will not suffer that the blood of his saints, which shall be shed by them, shall always cry unto him from the ground.”