What is the theme in Poirot Investigates by Agatha Christie?

Deception figures prominently in all these stories. Roger Rolf deceives his wife while having an affair with Lady Yardly whom he later blackmails. He arranges the robberies of the diamonds. Mrs Maltraverse deceives and murders her husband, having previously ensured that he insures himself. The Robinsons are deceived to rent a cheap apartment that turns out to be a plot for others to avoid a vengeance from mafia. Mr Shaw deceives the bank placing an empty package on the Olympia ship and pretending to steal it while defrauding the bank.