Poirot has just accused Dr. Sheppard of killing Roger Ackroyd. Dr. Sheppard laughs and says that Poirot is insane. Poirot then points out that Dr. Sheppard’s statement was the only proof offered for the fact that the window was locked. Poirot realized that Sheppard must have killed Ackroyd before leaving the study. Then he must have run outside, changed into Ralph’s shoes (which he’d kept in his bag), left prints on the windowsill, and rigged the dictaphone to play at 9:30.
In this chapter, readers begin to see how Sheppard has managed to pen an entirely truthful manuscript about the murder without giving away the fact that he’s a murderer: while everything he says in the book is the truth, he’s omitted a lot of information, or carefully worded his statements to avoid incriminating himself.
Dr. Sheppard protests that he had nothing to gain by killing Roger Ackroyd. On the contrary, Poirot guesses, Sheppard killed Roger to protect himself. He blackmailed Mrs. Ferrars, having gone into debt due to bad investment—and he hadn’t, contrary to what he told Poirot during their first encounter, come into a legacy. Then, when Ferrars died, Sheppard had to prevent Roger from learning the blackmailer’s identity. Poirot’s biggest obstacle in solving the case, he admits to Dr. Sheppard, was the phone call. The steward of an American liner—one of Dr. Sheppard’s own patients—had made the call from King’s Abbot station. Poirot telegraphed the man earlier and received a message, confirming that Sheppard asked him to phone him from the station.
In retrospect, it’s clear that Poirot was referring to Dr. Sheppard, not Ralph Paton, in Chapter 17: an ordinary, weak man, who’s driven to murder because of financial difficulties and a threat to his reputation. This would indicate that Poirot has been suspicious of Dr. Sheppard for a long time. It’s not clear if Poirot ever considered Sheppard a good friend, comparable to Captain Hastings, or if he only pretended to think so to trick Sheppard.
Dr. Sheppard tells Poirot that he’s weary of Poirot’s lecture. However, Poirot reminds Sheppard that he’ll tell Inspector Raglan the truth tomorrow—unless, for Caroline’s sake, Dr. Sheppard chooses an easier way out, such as an overdose. He suggests that Sheppard finish his manuscript. He also warns Sheppard not to try to silence him, as he silenced Roger Ackroyd. Sheppard smiles and says, “whatever else I may be, I am not a fool.”
Poirot is prepared to bring Dr. Sheppard to the police, and yet he’s also giving Sheppard another way out: suicide. While a police officer would be legally bound to arrest Sheppard, Poirot adopts a subtler, and perhaps more ethical strategy: to protect Caroline from the shock of learning that her brother is a killer, Dr. Sheppard can kill himself, and—it’s implied—Poirot will convince Raglan not to publicize the findings of the investigation. (Although it also seems unlikely that Caroline would simply accept that no killer had been found at all, not to mention how all the other murder suspects would react, and how effective Caroline usually is at discovering secrets in her town.)