Our favorite little Belgian detective's little gray cells are most offended by the actions of the ABC killer. Why? Because this is, to use his term, a "public" murder. Messr. Poirot is much more comfortable with "personal" murders where there the deed is done to act out one of those common (and personal between the killer and victim) human emotions like greed, lust, jealousy, desire for revenge, or a quest for power---but usually cash is king. In Mr. ABC Poirot encounters his first serial killer, and he is not happy. Freaked out is more like it, actually.
ABC is following the train schedule (the "ABC Guide") and committing murders in cities starting with A and working through the alpahebet. The victims are random (perhaps?) people whose last name starts with the same letter as the city. Oh, and he taunts Poirot by telling him in advance the name of city and the date of the killing. Mr. ABC is quite confident, you see, which makes Poirot even more apprehensive.
A masterful novel by probably the greatest mystery writer of all time at the top of her game.