Due to the nature of this story, spoilers are unmarked, read at your own peril.
Also bear in mind that to some extent some of these examples of idiocy are offset by the fact that it's still a 1930s British society we're talking about and that the characters could hardly have been Genre Savvy due to this very book being a Trope Maker.
In General
- The story takes place in an isolated location.
You'd Expect: They'd make sure nobody has weapons or poison and everyone is accounted for at all times the second they know what's going on.
Instead: They utterly fail to take basic precautions (Aside from Wargrave, Lombard, and Vera, depending on the version)
As a result: Way more people die than would be the case otherwise.
The Original Novel
- Brief in-universe example. Macarthur considers himself this for not realizing that Leslie's friendship with his friend was fairly obviously an affair waiting to happen.
- At one point Blore notes that several suspects had no opportunity to poison Mrs. Rogers. Wargrave retorts that someone could have entered her room with medication in the roughly 10 minutes Rogers spent clearing the dining room in the middle of the night.
You'd Expect: Blore (or anyone else) to point out how absurdly unlikely that is. How would the killer know when Rogers went downstairs as opposed to anyone else and why would they wait for an opportunity that could potentially never come?
Instead: They accept this as fact, and move along.
As a result: Everyone's a suspect.
It gets better: The murderer actually poisoned Mrs. Rogers by poisoning her tea. This never discussed as a possibility. - Ms. Brent wakes up early.
You'd Expect: She'd wake someone else up or hang out with Rogers.
Instead: She takes a nice relaxing stroll on the island by herself.
As a result: She avoids being murdered (probably because the killer had no idea she'd do something like that) but she is considered suspicious by everyone because of how careless she was. - Rogers (along with everyone else) knows there's a killer amongst them, killing them in accordance with a children's rhyme. The next verse of the rhyme involves someone dying while chopping wood.
You'd Expect: Him to not place himself in a really obvious position to be murdered, especially if it fits the rhyme as perfectly as possible. If he had to chop wood, he should have at least placed himself to be facing the door.
Or: He'd bring people with him to avoid being killed while alone.
Instead: He starts chopping up wood with his back to the door.
As a result: He gets easily murdered, just like the rhyme. - Lombard brought a gun to the island.
You'd Expect: He'd put that thing in a secure location when he realizes that there's a killer on the loose, especially since he already told two people he has it.
Instead: He leaves it in the literally most obvious place he could.
As a result: His gun is stolen. - Emily Brent isn't feeling so good after eating (!) and says she'll wait in the dining room for her dizziness to pass.
You'd Expect: Literally anyone to offer to stay with her.
Or: Armstrong to make sure she's all right (she's not feeling good after eating? Hmmm...)
Instead: Everyone leaves and goes to another room.
As a result: The murderer gets extremely lucky and gets away with another kill.
To make it worse: She's killed while the group is having a group discussion. It should not have been hard to make sure everyone was there. - Wargrave approaches Armstrong with a plan. Wargrave will fake his own death and Armstrong will pronounce him dead. Then Wargrave can spy on the other guests easily and figure out who the killer is.
You'd Expect: Armstrong to at least consider the possibility that Wargrave himself is the killer. If he is, Armstrong is an easy target and the others will be unable to stop the killer.
Or: For him to realize that the murderer would most likely see through that plan and murder both Armstrong and Wargrave right away.
Or: For him to realize that Wargrave would have a hard time spying on anyone in such confined quarters.
Or: He'd suggest that they "kill" Wargrave in a less "Judge" fashion to throw the killer off a bit more, perhaps screwing up his rhyme scheme?
Or: "Wait, isn't the next murder about a red herring? This seems suspiciously likely to make me a target."
Instead: He goes along with it because he believes a man as professional as Wargrave couldn't possibly be the killer.- So Wargrave's death is now faked, congrats Armstrong, what'll you do now?
You'd Expect: Stay away from Wargrave and act normal. He doesn't want people to realize Wargrave is alive.
Instead: He gets drunk and meets Wargrave on a cliff face, 100% allowing someone to follow him (like Blore nearly does).
You'd Then Expect: Him to stay on his guard, just in case.
Instead: He falls for the old "Look over there" trick and gets drowned.
As a result: Everyone is screwed and Armstrong is dead.
- So Wargrave's death is now faked, congrats Armstrong, what'll you do now?
- So as part of the above murder plot, Armstrong plants some seaweed in Vera's room to trigger her PTSD and make her scream as a distraction while they fake the murder. She does and Wargrave's murder is now faked.
You'd Expect: Lombard or Blore to realize that it is physically impossible for Vera to have committed the murder, given she was the focus of attention the entire time. It's impossible to be sure, but one could speculate that Lombard would give Vera the revolver or something for safekeeping. At any rate suspicion would be off Vera, possibly to the point of Lombard piecing together who the culprit is.
Instead: No one notices this incredibly obvious detail. - Blore thinks it's a good idea to go into the house that the killer is likely hiding in... BY HIMSELF! (He's hungry and neither Vera nor Lombard want to risk going into the house).
You'd Expect: "Wait, no, that's a terrible idea."
Instead: Despite both Lombard and Vera trying to talk him out of it, he thinks he can take care of himself and enters.
As a result: Blore gets murdered absurdly easily.
Bonus Points: He leaves Vera with Lombard, whom he somewhat suspects of being the killer. If Lombard actually was the killer, he could have killed Vera and shot Blore. - Lombard realizes that Vera is the last person standing aside from him, and thus must be the killer. He pulls a gun on her and...
You'd Expect: He'd shoot her right there and then, or at least tie her up or something.
Instead: Vera politely asks him to help her move the body, and he complies.
As a result: She steals his gun while they move the bodies.
To be fair: He doesn't have the gun out, and he's a "seasoned adventurer" and a bit of a chauvinist. He doesn't believe she could possibly get the drop on him, and it proves to be one of his fatal mistakes. - So now Vera has a gun trained on him.
You'd Expect: He'd convince her that she's going to need him alive to avoid being suspected and perhaps she should just tie him up or something. Or, point out the flaws in her logic of him being the killer, as he attempts in the 2015 miniseries.
Instead: He lunges at her head on while she has a gun trained on him.
Predictably: She shoots him. - Even the Police are not immune to this. As they sift through the clues, they utilize the journals and notes taken to establish that Armstrong, Vera, Lombard and Blore were the only ones standing. After careful examination, they discover that the bodies of Lombard, Vera, and Armstrong were tampered with after their deaths.
You'd Expect: "...So Blore must be the culprit!"
Instead: "...And Blore used to work with us. He was pretty unethical, so its unlikely he'd murder these people for Justice."
As a result: The case is unsolved until a random fisherman finds the killer's Message in a Bottle.
To be fair: Blore isn't the killer, but there's really no reason to assume it can't be him.
Added to which: They also rule Blore out on the grounds that people just don't commit suicide by trying string to heavy marble clocks and pulling them onto themselves. They somehow miss the point that there was no piece of string tied to the clock. In other words, there was a good reason to rule Blore out: the clock had to have been pushed by someone else, but they somehow manage to miss this.
The 1945 Movie
- The general is acting a bit insane, rambling about his wife. This causes people to think he might be the killer, if there is one.
You'd Expect: Someone watch him. There have been two suspicious deaths already.
Instead: He's 100% neglected and left to his own devices.
As a result: He gets murdered alone. - The group decides to vote one person out of the house, who must stay in the lumber shed (like this is a game show or something).
You'd Expect: At least a few of them would vote Armstrong, who had literally all the tools one would need to commit the first murder, or Blore, who flat out lied to them.
You'd Also Expect: They'd realize what a horribly stupid idea sending someone in the area where the next murder will likely take place is.
Instead: They vote out the butler. The guy doing butler stuff for them.
As a result: He refuses to make drinks for them. (NOOOOOOO!) And predictably, it gets Rogers murdered too. - A power outage occurs when Blore tries to work the generator. Armstrong and Wargrave were the only ones who didn't know that.
You'd Expect: The others would explain what happened.
Instead: They don't and Wargrave declares that this must be the work of the killer, to set up their next murder.
Since Wargrave is himself the killer, this is actually a way to make the doctor trust him more.
You'd Expect: Armstrong to bring up the power at some point, perhaps there's another explanation...
Instead: Nope.
As a result: The Disaster Dominoes start for the group when Armstrong fakes Wargrave's death.
At least: There's a reason for Armstrong to trust Wargrave in this version. - Blore sees Armstrong's corpse through his binoculars.
You'd Expect: He... would move? He would walk over there? He would not stand under a window for so long?
Instead: He just kinda stands dumbfounded until someone puts him out of his misery from the window above
Additionally: He chose to look in his binoculars while standing under a window for some reason.
And Wait A Minute!: What does dropping a piece of roof on a guy have to do with bears? - Wargrave's evil plan has come to fruition. Vera has (seemingly) shot Lombard and is the last person Wargrave plans to murder. He then drinks poison, telling Vera that as the last person alive, she'll be hanged for murder.
You'd Expect: He'd rethink his plan a little.
Instead: He goes through with it, not realizing that:
A) He used his own medication to kill a few people.
B) He told Vera everything about his plan, so instead of falsely blaming Lombard, she'll blame Wargrave.
C) If this is anything like the novel, several people kept journals and notes, making it that much easier to figure out what happened.
As a result: Vera would probably be able to pin everything on Wargrave and live, foiling his plan, even if it weren't for the fact that Lombard isn't even dead. (You can practically hear Vera laughing internally when she realizes he doesn't know that).
To be fair: Wargrave/Quincannon says that it won't really matter, as the only person left alive on an island of corpses will automatically be suspected as the killer. Also, he's a well-known judge with a high standing. It's not flawless, but it COULD work in his favor. - Lombard is not Lombard! He is Charles Morely, and he came in Lombard's place to figure out if the invitation had anything to do with Lombard's suicide last week.
You'd Expect: He would tell someone after the first body! If nothing else, the killer might ignore him as a possible target.
Instead: He doesn't until Vera holds him at gunpoint.
As a result: He is on the killer's list of targets and several people are suspicious when his luggage contains the initials C. M.
2015 BBC series
- As the dinner party ends, the record player lists its accusations. The guests all freak out, naturally. Emily Brent and Vera deny the accusations that they murdered by inaction. Armstrong says the doctor is always blamed when something goes wrong.
You'd Expect: People to focus. It doesn't matter if the accusations are true or not. They are trapped and in danger.
Instead: When Lombard admits the accusations against him are true, the others turn on him. He retorts that either he's among liars, or he's a Drama Queen embellishing a story.
The Result: Wargrave uses the distraction to poison Marston's drink with cyanide. - Vera steals Lombard's gun after they find the doctor's corpse on the cliffs. She starts screaming that Lombard was the killer, and is obviously frightened for her life. Lombard knows that Vera isn't U.N. Owen because she was out of the house when he found Blore's body after Blore entered the house first; there was no time for her to commit murder. Besides which, they most definitely found her unconscious in her room when the judge was shot; no one knows that the judge faked his death. Lombard starts to reason with Vera, saying neither of them is the killer and they are still being hunted.
You'd Expect: He would lie down on the beach, hands over his head, and reassure Vera that he's no threat. Or point out that if he were the killer, he would have shot her the moment they found the doctor's body since by then it would be her turn in the rhyme next. Or, going by the version with Charles Morley, order her to shoot him because if they're still being hunted then the real killer is watching them. Mind none of this may have saved him because Vera is beyond reason, but at least he would have tried.
Instead: He keeps saying, "Give me the gun, Vera" after trying to say they're hunted.
The Result: Vera shoots him repeatedly as he approaches her. He bleeds out on the beach, as she walks back to the house. - In the climax, Vera has shot Lombard and prepares to hang herself when she sees the noose in her bedroom. Then Wargrave appears, alive and well; he reveals he is the killer. Vera, balancing on a tipped chair and slowly being suffocated, starts begging for her life.
You'd Expect: She would maintain her pretext of innocence: she didn't kill Cyril, she was too late to swim after him!
Instead: When she suggests framing a dead Lombard for the crimes, she lets it slip that she did kill Cyril and lied to the coroners before. She does this to a man who has murdered an entire party of killers that evaded the law.
The Result: Wargrave humors her for a minute, before pulling the chair away.
