why did christy make this mystery almost impossible to solve

And Then There Were None can be criticized as an unfair mystery novel. In a standard mystery story, a crime is committed, a detective comes in to solve the crime, and the reader follows along with the detective, learning everything the detective learns, and collecting clues that would theoretically enable the reader to guess the identity of the killer. At the close of such a mystery, the detective usually gathers the remaining characters together, reveals the identity of the murderer, and explains how the crime was committed.

And Then There Were None breaks all of these rules. First, there is no tidy arrest: the murderer gets away with his crime, and we discover his identity only because he leaves a confession behind. The only outside detective is a policeman who arrives too late to accomplish anything and who is utterly baffled by what has happened. Most unconventionally, the novel deceives us: we believe that Judge Wargrave is dead, and so we no longer suspect him. In fact, he is still alive, and he is the killer.

In some ways, however, And Then There Were None is a very conventional murder mystery. Ten people are isolated and cannot escape; suspicion falls on all of the characters; red herrings abound; a satisfyingly neat ending is produced in which the murderer’s actions and motivations are explained, and the pieces of the puzzle fit together tightly. Although Judge Wargrave is the killer, he also plays the role of the detective, unmasking the criminal—himself—at the end of the novel and explaining how everything transpired. Though Christie breaks some rules, she does so to make the story all the more suspenseful.