24. What was the motive for the crime?
From Quiz Evil Under the Sun
Answer:
Money
Although Arlena Marshall had certainly inspired a great deal of jealousy, resentment, and passion, in the end she was murdered simply (and quite cold-bloodedly) for monetary gain. Patrick, a ruthless fortune hunter, had bilked her out of nearly all of the money she had inherited from a wealthy, elderly admirer by persuading her to hand him over large sums for "investment". Realizing that there would be hell to pay when Arlena's husband discovered the swindle, he coldly planned to murder her before she would have a chance to tell Kenneth where the money had gone. To explain the disappearance of the money, Christine fabricated a story about having overheard someone blackmailing Arlena. Poirot found this story hard to swallow (Rosamund Darnley also found it "incongruous"); Arlena simply wasn't the type to be successfully blackmailed. Moreover, it was she who had been murdered; why would the blackmailer kill the source of his money?
The murder of Arlena Marshall was the second (possibly the third) murder which Patrick had committed for financial gain. Previously, under the name Edward Corrigan, he had murdered his wife Alice Corrigan in order to collect on her insurance policy. In this crime, he had been provided with an alibi by a woman named Christine Deverill (Christine Redfern), who claimed to have "found" the body before the murder had actually been committed. Strangulation was the modus operandi in both cases; Poirot believed that Patrick employed this method because he enjoyed the act of killing; it was he, not Arlena Marshall, in whom Poirot instinctively (and correctly) sensed the presence of evil at the beginning of the novel.