When is the killer discovered?

The killer, Judge Wargrave, actually isn't revealed until the epilogue. The Epilogue and the closing letter chapter serve as an overview of the previous sixteen chapters, to sum up the facts and clues of the murders on Indian Island, and to offer solutions to the mystery. While the previous chapters of the novel sought to disguise the essential facts of the case and to disrupt any meaningful interpretation of the story, these chapters seek to clarify through a restatement of the facts as seen through the eyes of the Scotland Yard detectives, and then through a reevaluation of the facts through the eyes of the killer. We see in these two evaluations where the detectives, and the reader, have gone astray in their assumptions regarding the murder.

It is important to note the change in voice in the final chapter. While the first seventeen chapters of the novel are written in the third person, the final chapter is written in the first person, from the point of view of the killer, Mr. Justice Wargrave. This change in perspective is necessary because it highlights an essential element of the detective novel: the narrator is always unreliable. In this case, the narrator had limited omniscience. The reader could observe characters and even hear their conscious thoughts, but the narrator was never able to relate facts outside of the limited purview of the murdered guests. Thus, the narrator was not able to know that Wargrave faked his own death or that Armstrong was unwittingly complicit in helping the murders take place.