Complete Monster: Michael Garfield, from "Hallowe'en Party", is a very handsome man who is actually the lover of Rowena Drake and the caretaker of her garden. Together, the two are responsible for murdering six people; however, whereas the latter actually did love the former and her two children from her deceased husband, Michael seduced her so as to get at her garden and her aunt's money, expressing disgust with her appearance to Poirot, and is shown to be a shameless flirt. In the past, Michael had killed Rowena's husband, which she blamed on reckless teenagers, and helped Rowena poison her aunt, who is revealed to have disinherited her because of the affair, instead leaving everything to au pair Olga Seminoff, when her will is read. To remedy this, Michael had Leslie Ferri clumsily forge a will—killing him afterwards—which Rowena switches with the real one before she kills Olga when confronted, with him burying her body. When he realizes that Miranda Butler, his daughter from his affair with her mother, had seen him, he told her it was a sacrifice, making her think of it as a pagan ritual sacrifice. He returns after Rowena drowns young Joyce, Miranda's friend, when she says she saw a murder at a Hallowe'en party. He kills or tries to kill people in his way, including his own child, this last one just stopped by Poirot. When Poirot reveals Michael's and Rowena's crimes, Michael shows himself to be a greedy narcissist who planned to run off with Rowena's money to purchase an island and create his own garden, caring only for his own desires and showing Rowena his true feelings about her.
Robin Upward from Mrs. McGinty's Dead. On the one hand, he murdered an old woman and framed an innocent man purely to protect his reputation. On the other hand, he's extremely pathetic and his Villainous Breakdown at the end when he's screaming at people not to look at him shows that he's never really gotten over his massive mommy issues.
Norma Restarick in Third Girl. Abandoned, betrayed or just plain let down by her father (both of them, actually), her mother, her lover, her nanny, her uncle and her stepmother, but it's easy to understand why they keep doing it, as she's unpleasant and neurotic to the point everyone (including her) believes that she is capable of murder, and she's even starting to suspect herself when her old nanny is found dead after she had an absence at an alcohol-fueled party. It then gets turned on its head when you realize that she's only that way because the childhood traumas her parents inflicted her have carefully been reawakened through constant manipulation by her would-be murderer, in order to gaslight her. The reason why they could get away with it ? Every single "innocent" person in her life was either too carefree, jealous, condescending or just plain selfish to notice or care.
Narm: If a Poirot story featured a children's rhyme, then the Suchet adaptations would invariable try and use them as a creepy leitmotif.
One, Two, Buckle My Shoe features two girls playing hopscotch while singing the titular rhyme as a recurring theme. Unfortunately, while the girls are clearly cheerful and having fun, their voices are dubbed over by grown women trying to sound like young children while also singing the rhyme in a slow and creepy manner. And this is heard repeatedly throughout the episode.
"Hickory Dickory Dock" features that rhyme being sung in unusually intense and hushed tones as a leitmotif whenever something dramatic happens on screen.
"Hallowe'en Party", like "Hickory Dickory Dock", would use the chant that accompanies Snap-dragon to signal dramatic developments seen on screen. The original novel actually didn't feature the chant, instead depicting the children playing the game yelling out in delight and in pain as they either grabbed a raisin or got burned by the flame.
Narm Charm: The end of How Does Your Garden Grow features the culprit going from "normal" to insane in about three seconds, overacting for the rest of the scene to a hilarious degree. It's so hilarious and unexpected that you can't help but be delighted during the scene.
Narrowed It Down to the Guy I Recognize: The series' Long Runner lifespan (1989 to 2013), and the cast of characters from many different generations of British television and film leads to a general aversion of this effect. For example: In Appointment with Death, when one sees that the victim's husband is played by Tim Curry, one starts to suspect him only to have his son being played by Mark Gatiss and John Hannah playing their doctor friend. Sad Cypress seems to be aiming for this, featuring Paul McGann and Rupert Henry-Jones among the guest cast (neither of them did it). Emily Blunt shows up in "Death on the Nile" and is the victim. Naturally, this only applies to viewers who hadn't read the books first.
Paranoia Fuel: "Death in the Clouds", because being pricked with something poisonous by a passer-by could apply to any plane, train, or bus journey even today. And similar things have happened in real life.
Replacement Scrappy: After regular comic-reliefs Inspector Japp, Miss Lemon and Captain Hastings were Put on a Bus for unknown reasons, numerous Scotland Yard inspectors sharing Japp's mustache, Iconic Outfit and accent, and often described as old friends of Poirot, began to appear (one of them, Inspector Spencer, even appeared several times). A faithful valet, George, was also added and took on part of Hastings' and Miss Lemon's roles. None of them were very well-liked. Japp, Miss Lemon and Hastings do appear again near the end of the series though, while the latter is also an important character in the series finale.
George, at least, is from the books and short stories, where his main traits are being an extremely efficient butler and a bit of a snob (Poirot puts his intricate knowledge of people's social standing to good use). There are a variety of police inspectors who appear in the source material (Japp having had far fewer appearances than in the series) but the ones in the show rarely even have the same names as them, let alone anything resembling their description.
Strawman Has a Point: On both sides in this version of Murder on the Orient Express. Poirot is correct in that people should always try to choose law and order over vigilantism and anarchy. However, the passengers retort that they already relied on lawful means to punish Cassetti, which only resulted in failure and further grief for them.
They Changed It, Now It Sucks!: The adaptation of The Murder of Roger Ackroyd has been criticised as a disappointment in this regard. The Twist Ending in the novel depends on two concepts - the Unreliable Narrator and the fact that the killer is The Watson. The first is not especially difficult to pull off, but would merely look out of place since no other episode in the series uses it; the second is undermined because they put Inspector Japp in it and he ends up fullfilling the same role, meaning the killer is reduced to just another suspect, and consequently the story is just another murder mystery. Nearly every Agatha Christie fan thinks this was a poor adaptation.
Unintentionally Unsympathetic: Downplayed in Three Act Tragedy with the murderer, Sir Charles Cartwright. While the episode doesn't whitewash him for his crime, it really tries to elicit him some sympathy in a How the Mighty Have Fallen / Alas, Poor Villain / Love Makes You Evil kind of way. The problem is that Sir Charles Cartwright may be one of the worst murderers of the Poirot stories, up there with the culprits of The ABC Murders or Evil Under the Sun. Indeed, his first murder was simply done because he wanted to do a rehearsal. Yes, a rehearsal, in order to execute flawlessly the crime he wanted to do. He didn't do this with random people either; he knew that the victim of this "rehearsal" was going to be one of his friends or acquaintances. He then proceeded to murder his childhood friend in order to cover the fact that he had a crazy wife with whom he couldn't divorce, so he could marry the woman he loved. Finally, he murders a stranger, a female patient at Strange's sanatorium, simply to create a false lead. And, after all this, the guy has the nerves to try to lie to his so-called true love and to blame Poirot for exposing him?
Invoked in Murder on the Orient Express when Poirot and a couple of the passengers witness a stoning of an adulteress in Turkey and Poirot excuses it with this trope; it comes back to haunt him later when the girl who saw it and angrily disagreed with him asks him how he could stand back and let that happen but (in this version) is revolted by the vigilante justice of the murderers, despite the Asshole Victim being far more deserving of his fate than the adulteress was. He fails to give himself a satisfactory answer.
The club where Major Eustace is interviewed in "Murder in the Mews" dabbles with this, with the waitstaff and lounge singer dressed in Asian farmer hats.
Cust in The ABC Murders. It's really hard not to pity the poor guy, not only because of his frequent blackouts, but also the emotional torment he suffers when he becomes convinced he committed the murders. Clearing his name is one of Poirot's greater triumphs.
Gustave in The Labours of Hercules. His desperate devotion to the killer leads him to commit suicide rather than betray the killer's identity. Even Poirot feels dismayed at Gustave's death.
Eileen Corrigan in Taken at the Flood. From the very beginning, she was a normal Catholic farm girl and parlor maid of Rosaleen Cloade, until David Hunter, whom his sister Rosaleen had excluded as his "first love", seduced Eileen, raped her, and made her pregnant; afterward he performed an induced abortion on her, then broke her will by promising her salvation if she followed his orders and "the fires of hell" if she did not. She was then forced to pose as his sister Rosaleen, whom he had just slaughtered along with her husband and his entire family by blowing up the house in a surprise bomb attack. As if that was not enough, she was repeatedly bullied and denounced as a slut through Kathy Cloade-Woodward's phone calls and induced by David into attempted suicide by morphia overdose (and she would have been dead had Poirot and Lynn Marchmont not arrived in time, had Dr. Lionel Woodward (a morphine addict) not stolen some of her morphine, and had the Spared by the Adaptation trope not come into play). After all she's done by believing that she has been "cut off from the mercy of God", the poor girl needed a hug so badly.
James Bentley in "Mrs. McGinty's Dead"; he's a nice guy who unfortunately isn't terribly smart and is a bit of a momma's boy. He's also just lost his job and is in trouble with his landlady, which makes it VERY easy for the murderer to frame him for the crime. Ultimately, it's only because the investigator has second thoughts and asks Poirot to reinvestigate the case that Bentley is saved and the true killer exposed.