What do we learn about Dr. Sheppard in the last chapter?

In the final chapter, Dr. Sheppard's true self peeks through – defeatist, certainly, but also somewhat smug about his accomplishments as well as his writing skills (“I am rather pleased with myself as a writer” (p. 284)), and entirely remorseless for his crime. Indeed, he expresses only the vaguest concern over what his suicide will do to his sister (“I should not like Caroline to know. She is fond of me, and then, too, she is proud… My death will be a grief to her, but grief passes” (p. 285)). He soon passes from any feelings towards Caroline to rueful frustrations over Poirot’s involvement. He seems curiously detached from the entire event, as though manifesting his defeatism as objectivity. Mostly, he uses the final chapter to fill in the last details that the reader (and Poirot) need to understand the entire crime – a satisfying conclusion for those looking to fully comprehend the entire mechanics of the crime, but less satisfying for those readers hoping for an emotional apology from the murderer.