Unlike the Alibi which is typically based on Aristotelian logic, the MOTIVE lies in a fuzzy area of unpredictable human behavior.
Still, it is possible to classify the Motive from a structural perspective and to identify corresponding Christie plots.
Can you provide additional comments on variations of the MOTIVE theme in her plots?
- Motive for a murder can be either obvious as in "A Pocketful of Rye", or obscure as in "Lord Edgeware Dies" or "Sad Cypress".
- The standard motives for murder are - greed, fear, love/hate, insanity of various types (antisocial personality disorder, megalomania ...). There are so many plots around one or more of these different types of motives - consider "The Mysterious Affair at Styles", "Cards On the Table", "Five Little Pigs", "Easy to Kill", "Crooked House" as examples.
- Poirot discusses the aforementioned types of motives with Hastings when speculating about the possible motive behind the random killings in "The ABC Murders". In conceiving the plot, Christie perhaps wondered if random murders are necessarily "insane acts", and whether there can be something like "deliberate randomness". In response, she created a plot where the perpetrator creates a haystack to hide a needle in. Further, since a perfect plot must not leave loose ends, there is even a fall guy set up as the insane murderer.
- Different Motives may apply to different characters in wishing the victim dead. But exactly which one of them was willing AND able to take the final step? "Hercule Poirot's Christmas" is one such title.
- Alternately, different characters may have the exact same motive to kill. " But exactly whodunit?" becomes the question. "And Then There Were None" is an example.
- Having structured many plots around #4 and #5, the logically inclined Christie naturally had to ask herself: Can there be an exception to the common premise? Does there have to be one and only one perpetrator of the actual act? She did take up that challenge, and came up with "Murder on the Orient Express". Despite its popular success, the mystery is too synthetic for my liking, and seems to be written primarily to satisfy a personal need to plan a plot around a theoretical exception. The plot to me is unnatural and not believable.