There is a small wrinkle in the story of Omer’s overthrow. He is not kept captive. He and his family (save Jared, who had rebelled) traveled “many days, and came over and passed by the hill of Shim, and came over by the place where the Nephites were destroyed.” Having also passed Cumorah, they stop at Ablom, on the seashore. Moroni sees Omer as a righteous king fleeing along the same basic path as did the later Nephites.
The rest of the story continues to describe the evils of secret combinations. Secret combinations placed Jared on the throne, but Akish (the one who had made the oaths) was not satisfied. He did what later Gadianton secret combinations would do. He murdered Jared “as he sat upon his throne.”
Moroni lectured on the evils of secret societies at the end of Chapter 8, and now he reiterates those evils. “So great had been the spreading of this wicked and secret society that it had corrupted the hearts of all the people.”