To show why A.B. Cust is the murder suspect in Agatha Christie's The A.B.C. Murders, describe the facts that point to and against Cust being the murderer, Cust's possible motives for each murder, why Cust would write letters to Poirot before each murder, and how Cust could possibly profit from each of the murders.
Facts pointing to Cust being the murderer in The A.B.C. Murders include the letters to Poirot signed with his initials and the stocking salesman seen near each murder. Cust has blood on his sleeve; he has a box of ABC railway guides, a typewriter, and paper in his room but has an alibi for one murder. A madman would not need motive or to profit from killing. The letters cast suspicion on Cust and delay delivery of the "C" letter.
First, the murderer writes anonymous letters to Hercule Poirot, taunting him with the crimes that he is about to commit. He signs the letters "A.B.C.," which is both a reference to the diabolical plot of killing people one by one as he works his way through the alphabet and also a way of throwing suspicion on Cust, whose name, Alexander Bonaparte Cust, also can be abbreviated as the initials A.B.C.
Second, Franklin Clarke deliberately intends to throw suspicion on Cust, who is innocent. He "hires" Cust as a travelling salesman of silk stockings. On the day of each murder, a man selling silk stockings was seen in the vicinity.
Third, Cust is an epileptic who suffers bouts of short-term blackouts following a head injury he received during WWI. Cust cannot recall his whereabouts during the last murder, and he was found...
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