In Chapter 27, Dr. Sheppard at last admits that he is the killer and explains how and why he did it. Even as he takes responsibility, he notes the situational irony that many other people seemed to implicate themselves in some way over the course of the investigation:
I wish I could have known beforehand that Flora was going to say she’d seen her uncle alive at a quarter to ten. That puzzled me more than I can say. In fact, all through the case there have been things that puzzled me hopelessly. Everyone seems to have taken a hand.
Sheppard was especially "puzzled" when Flora claimed to have gone into her uncle's study at a time when Sheppard knew the man to be dead. Clearly she was lying, but why? Sheppard knew that he worked alone to commit the murder, so he could not imagine why Flora would lie and place herself in a more guilty position. Sheppard notes that Flora was not alone in muddying the case: "Everyone seems to have taken a hand," he writes, "puzzling" him and even covering up what really happened.
Sheppard's surprise at this situational irony is ultimately his downfall. If he had understood that innocent people often behave as if they are guilty, he may not have relied so heavily on framing Ralph Paton. Poirot, on the other hand, insists over and over that even people innocent of the crime in question always have secrets and often look very bad. This attitude pushes Poirot to investigate every possible suspect, especially the people who look the least guilty. Sheppard does not initially come off as a viable or motivated suspect, and that is precisely what makes Poirot suspect that he might have something to do with the murder.
