It’s 5 a.m., and Dr. Sheppard has just finished his manuscript. He pities Roger Ackroyd, and wishes Roger had read the letter when Sheppard gave him the chance. Or perhaps, subconsciously, he urged Roger to read the letter because he knew this would make Roger unlikely to read the letter.
Christie presents the book we’re reading as a manuscript, penned by Dr. Sheppard and completed in his last hours of life. One reason that the reader may have doubted that Sheppard could be the killer is that he insisted that Roger read the letter, which would naturally imply that Sheppard wasn’t the blackmailer. Sheppard’s explanation is that he pitied Roger and didn’t want to kill him. Had Roger read the letter and learned that Sheppard was the blackmailer, he might have screamed for help, or put up a fight (rather than sitting in his chair and allowing Sheppard to stab him in the back), rendering Sheppard’s complicated murder plot unworkable.
Dr. Sheppard says that he used a dagger to kill Roger Ackroyd as an afterthought. He’d brought his own weapon, but decided to use one that couldn’t be traced to him. Sheppard had planned to murder Roger as soon as he heard of Mrs. Ferrars’s death. When he ran into Roger in the street, he half-expected Roger to have already learned he was the blackmailer. Thus, he took precautions before coming to Roger’s house.
Another reason readers may have doubted that Dr. Sheppard could be the killer is that he used a weapon from the Ackroyd house, suggesting that the crime was committed by someone who had regular access to the knife. However, Sheppard explains that he simply substituted one weapon for another.
Dr. Sheppard says he is proud of himself for misleading readers, particularly when describing the time of the murder. He simply omitted everything he’d done between 8:40 and 8:50, including setting up the dictaphone, which Roger had asked him to fix, and which had been rigged like an alarm clock to go off at 9:30. Later that night, he was able to do “what little had to be done”—namely, returning the dictaphone to his bag and pushing the chair back.
Dr. Sheppard was one of the first unreliable narrators to appear in a detective novel; in fact, introducing the unreliable narrator to detective fiction is probably one of Christie’s most important contributions to the genre. Nowadays, narrators can’t be trusted in detective novels—they’re just as likely to be suspects as any of the other characters.
Dr. Sheppard must now contemplate his “way out.” To save Caroline from the truth, he says, he’ll take a sleeping pill—perhaps Veronal, creating a kind of “poetic justice,” since Mrs. Ferrars killed herself in the same way. He concludes, “I have no pity for myself,” but adds, “I wish Hercule Poirot had never retired from work and come here to grow vegetable marrows.”
The novel comes to an end with Sheppard planning to kill himself, and Poirot planning to conceal the truth from Caroline. With his signature blend of intuition, logic, and empiricism, Poirot has discovered the truth. But Poirot isn’t just interested in truth—he’s also committed to justice: preventing the truth from causing distress to other people, such as Sheppard’s loving sister.