The narrator tells the reader that "the symmetry of form attainable in pure fiction" cannot be had in a more truthful tale such as this one. Thus, he will not be able to give his story a good ending. He says that the story's final three chapters serve as "something in a way of a sequel" to the main story of Billy's death.
The narrator takes great care to distinguish his story from fictional tales, and, again, this serves to separate the narrator from Melville. Melville wrote the fiction that is Billy Budd. The narrator was part of that creation, and lives in the world of the novel. That means that for the narrator, the story is true; but it also means that the narrator is just as fallible as any of the other characters in the story and the narrative he relates should therefore be treated with the same skepticism you would treat any other story you might hear from a friend or were you, say, serving on a trial as a jury. The narrative we read in Billy Budd is the narrator's version of the story, the narrator's interpretation of the motivations of the characters based on their actions.
After Billy's death, the Indomitable engaged in combat with a French vessel named the Athéiste. During the fight, Captain Vere was hit with a musket ball and mortally wounded. Just before dying, he uttered the words, "Billy Budd, Billy Budd," though the narrator says that the words were not said remorsefully.
In the final moments of his honorable death, Captain Vere remembered his dear comrade Billy. But the narrator specifies that his words were not remorseful—Vere still felt that he made the necessary decision in executing Billy. The death of Vere also removes the last true participant in the story. There is no one left to go to if one want's to get "primary research" regarding the story of Billy Budd.