Complete Monster: Franklin Clarke, aka "ABC", wishes to inherit his brother's wealth, but is aware that he would be the obvious suspect. ABC hits upon the fact that his brother has an alliterative name and lives in a town that begins with the same letter. With that in mind, ABC proceeds to kill Alice Ascher in Andover, Betty Barnard in Bexhill, and Sir Carmichael Clarke in Churston, creating the illusion that a Serial Killer is on the loose, in an effort to hide his murder-for-profit. ABC also picks out a patsy in epileptic Alexander Bonaparte Cust, and preys on his insecurities, convincing Cust that he has been committing the murders during his blackouts. Committing one last murder in Doncaster, ABC slips the knife into Cust's pocket, and then calls the police, intending that Cust either hang, or end up in a mental ward, while he walks away with the money.
Fridge Horror: Had the killings kept going on instead of having Poirot catch ABC after D, the case could have gotten very personal: Poirot's Friend on the Force is named James Japp.
Harsher in Hindsight: The killer sends taunting letters announcing the dates in which he is going to kill someone. One letter announces his next killing will be on September 11th! Hastings even says that, "I shall, I think, remember that 11th of September all my life. Indeed, whenever I see a mention of the St. Leger my mind flies automatically not to horse-racing but to murder."
The Woobie: Alexander Bonapart Cust is a simple, unassuming guy who gets a lot of misery from his peers due to his grandiose name. Then, he got drafted in the war and received an injury that cause him to become epileptic, suffering from frequent blackouts. The killer then used Cust as a scapegoat, and manipulated him to be on the crime locations on the days of the killings. This cause the poor guy to actually believe that he is the murderer, and that he has committed all the crimes during his blackouts.
The BBC series
Ho Yay: In the BBC adaptation, Franklin's final speech to Poirot feels like this throughout, but when he gets to the ending section it positively reeks of Ho Yay, with the ending being Franklincaressing Poirot's scar and saying he's going to miss him.
They Changed It, Now It Sucks!: The general reaction to the 2018 BBC adaption, which makes a lot of changes to the material for the sake of being Darker and Edgier which are unnecessary like getting rid of Inspector Japp and Captain Hastings and makes it so bleak it's almost unwatchable. And that's not even getting into the replacement of Poirot's iconic moustaches, with a generic looking goatee beard. Between this, and his Adaptational Personality Change, there's little of the original character left at all.
