Agatha Christie published The Murder of Roger Ackroyd in 1926. The author maintains that the idea of designating the story's narrator as the killer originated from her brother-in-law James.
At the time of her story's publication, it was generally not the practice of authors to designate the narrator as the guilty party. At the time, readers were accustomed to trusting the objective voice of an impartial observer of events. Christie's introduction of a "guilty narrator" shocked readers, and some accused her of "breaching the reader's trust."
However, others believed that the breaking of this seemingly iron-clad "rule" lent suspense to the story. Today, many literary experts believe that The Murder of Roger Ackroyd is one of Agatha Christie's most thought-provoking stories.
In this story, a seemingly "normal" narrator who is capable of killing leads us to question our own goodness. Essentially, the idea that every human soul is capable of evil is a discomfiting one. This is precisely what made Agatha Christie's story so controversial for its time.
In the story, the narrator is the upstanding Dr. Sheppard, a well-respected physician in the village of King's Abbott. When Mrs. Ferrars commits suicide, he is sent for. Naturally, the woman's fiancé, Mr. Roger Ackroyd, is devastated by her untimely demise. Roger also confides in Dr. Sheppard that someone was blackmailing Mrs. Ferrars prior to her suicide.
Essentially, Mrs. Ferrars had murdered her husband and was terrified that the truth would get out. In her suicide note, she apparently confessed the identity of her blackmailer to Roger Ackroyd. When Roger is murdered, stabbed by a knife from his collection, Hercule Poirot is called in to solve the mystery.
From his investigations, Poirot deduces that the killer is Dr. Sheppard, who blackmailed Mrs. Ferrars. At the end of the story, Dr. Sheppard commits suicide in order to avoid being incriminated for Roger's murder.
In making Dr. Sheppard the killer, Agatha Christie may have "breached the reader's trust," but she also helped popularize the use of the unreliable narrator in modern fiction.