Without Judge Wargrave's manuscript explaining what happened, nobody would ever have known who murdered the people on the island. It would have been an unsolved mystery. It represents the perfect crime.

We know this because the Assistant Commissioner and Inspector Maine do a thorough investigation of what happened. They interview the islanders and examine the case from every possible angle and cannot figure out what really occurred. The identity of the murderer remains a complete mystery. The novel ends with a question the Assistant Commissioner poses: "who killed them?"

It is not until the manuscript arrives at Scotland Yard, having been retrieved in a bottle from a boat called the Emma Jane, that the detectives and readers find out what really happened. Here, Judge Wargrave reveals that he was the mastermind behind all the deaths. He was terminally ill and decided to kill all these people because they had gotten away with crimes or immoral deeds that led to a death of another person. They had fallen through the cracks of the criminal justice system.

Judge Wargrave fakes his own death while some of the victims are still alive. None of them are medical professionals, so they don't want to have much to do with the body and they don't know he is not really dead. Later, the judge arranges to shoot himself, so that it looks as if he were murdered like everyone else.