He is being used, set up as a cover for the real murderer, who is the brother of the third victim. Franklin Clarke has set up the whole scheme, including hiring Cust to be a salesman and the fall guy, in order to kill his brother so as to not be disinherited. The motive is, as it is so often, money.

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 with blood on his sleeve and a knife.

Fourth, when the police search Cust’s room, they find a box of ABC railway guides that are identical to the guides the killer leaves at each crime scene. They also find a typewriter and paper that is identical to those that the A.B.C. murderer used in his letters to Poirot.

Fifth, Poirot doubts that Cust is guilty, in part because he has a solid alibi for the second murder at Bexhill.

In terms of Cust's possible motives for each murder or how he would profit from these crimes, Franklin does not need to show that Cust has motive or would profit. Franklin has planned the killings to look like the work of a maniacal serial killer to mask the true motive, which is the killing of his brother, Carmichael Clarke, the third victim.

Franklin devises an elaborate scheme to murder a series of people in what looks like a madman’s serial murder plot to kill people in alphabetical order based on their first and last names and location of the crime. Thus, his first victim is Alice Ascher in Andover (AAA), then Betty Barnard at Bexhill (BBB), and then Carmichael Clarke in Churston (CCC). If the murderer were a madman, he needs no motive for the killings, including a profit motivation.

Franklin (not Cust) writes letters to Poirot about the crimes as a way of establishing the pattern, throwing suspicion on Cust and giving himself time to kill his brother. Poirot wonders why A.B.C. would write to him rather than Scotland Yard, but it is because Franklin deliberately sends the “C” letter to the wrong address but one that looks like Poirot's. This causes a delay in delivery of the letter so that Poirot cannot warn Carmichael Clarke in time.

In Agatha Christie’s detective novel The A. B. C. Murders, almost all the evidence that the police and Hercule Poirot uncover points toward the guilt of Alexander Bonaparte Cust. His initials are A. B. C., matching those of the murderer who writes the letters to detective Hercule Poirot that precede the murders. Cust sells silk stockings, which matches a description of the possible murderer as someone who sold stockings to some victims. When he is identified as being present at the crime scenes, a typewriter found in his room proves to be the one used in the letters to Poirot. Because Cust has an alibi for the time that one of the murders was committed, the case against him is far from watertight.

The fourth murder should have corresponded to the letter d, but an apparent coincidence spared the man whose name started with d, and a man with an e name is killed instead. Cust was present at the cinema where this murder occurred and later finds that he has blood on his sleeve and a bloody knife in his pocket.

When he is identified as the likely suspect, Alexander Cust provides a long story about a job for which he was hired, in which capacity he was furnished with a typewriter. However, his inability to produce corroboration casts further suspicion. He claims that his presence at or near the location of some murders is connected with this employment.

The biggest point in his favor is his alibi for the time of the b murder. Having struck up a casual acquaintance, he ended up playing dominoes with a man named Strange, who is absolutely certain that Cust was with him that night. Cust worries that he might have committed the other murders, because he suffers from blackouts related to epilepsy.