A convenient way to deal with someone you want dead is to simply not save their life when a situation arises in which their death is inevitable.
Killing is messy. You have to deal with those pesky murder charges, or go to the effort of engineering a convenient "accident" to avert suspicion, or clean up the crime scene to hide your involvement. But as fate would have it, if your foe winds up in a fatal position and you are their only means of survival, all you have to do to kill them... is nothing at all.
In Real Life, this concept is called the duty to rescue. According to The Other Wiki, the failure to offer help for those in need is rarely considered a crime, but there are some countries where people are obligated by the law to come to the aid of those in life peril. In France for example, abandoning a helpless person can earn you a prison sentence of up to five years.
Compare Make It Look Like an Accident and Suicide, Not Murder. Contrast Accidental Murder. If the victim is murdered actively by a character and another character does not intervene, it's a case of Accomplice by Inaction. Contrast/Compare Failure-to-Save Murder, where someone is held responsible for a death, because they tried and failed to prevent it and Bystander Syndrome when people don't help the victim for any reason. Is often the Start of Darkness for a character. This is often a favored tactic of the Technical Pacifist.
As a Death Trope, all spoilers will be unmarked ahead. Beware.
open/close all folders
Batman: In issue #633, Robin (Stephanie Brown) dies due to torture and Batman later discovers that Dr. Leslie Thompkins deliberately withheld treatment that could've saved her life but chose not to in order to teach the kids of Gotham a lesson about superheroing. This was retconned into Thompkins making Batman think that Stephanie died, but she didn't really die.
Marshal Law: At the climax of the "Kingdom of the Blind" storyarc, Law is very much capable of helping the Private Eye up rather than let him fall to his death. Watching from across the room, Law jokingly insists he "can't quite reach" as the Private Eye struggles and eventually falls.
Film - Animated
The Emperor's New Groove: As Kuzco and Pacha cross a rickety old bridge on their way to the palace, Pacha falls through and ends up tangled up in the ropes. Rather than help him up, Kuzco leaves him there, saying that it's better than imprisoning him in a dungeon as per his original plan. This backfires immediately when he too falls, forcing the two of them to work together to save themselves.
Frozen: Attempted by Prince Hans when he first chooses to withhold a potentially life-saving Almost Kiss from Anna, then leaving her to succumb to her frozen heart. You could argue that he also sped up the process by extinguishing flames, but ultimately, it was a choice not to save, rather than to kill. His kiss probably wouldn't have saved her any ways since he didn't love her.
Toy Story 3: Rather than save the toys from being roasted by the incinerator by pressing the emergency stop button to shut off the conveyer belt, Lotso instead decides to abandon the toys even after they risked their lives to save him.
Gladiator: After Maximus disarms Commodus in the arena, Commodus immediately starts demanding one of the surrounding Praetorian Guard to give him a sword. If he hadn't recently and publicly dishonored his own royal guards, they might have.
From Dr. Isaac Asimov's I, Robot collection, "Little Lost Robot": This is a concern of Dr. Susan Calvin. A robot has been built with a modified First Law, which in its case permits a robot to allow a human to come to harm via inaction. Calvin posits a situation where a robot with this modification can commit murder - by dropping a heavy weight above a human, knowing that its quick reflexes will allow it to catch the weight in time to not harm the human; but then having dropped the weight it has the ability to decide not to catch the weight.
Redwall: Ungatt Trunn dies when, after surviving being thrown into the sea with a broken back, finds himself stranded as the tide comes in. Then his much-abused former seer shows up to gloat, not doing a thing to get him out of the rising water.
In Raven In The Foregate, one of Ellis Peters' Brother Cadfael mysteries, it turns out that the victim wasn't murdered (by being hit on the head and thrown in the river) at all. The man who was the prime suspect simply didn't help him when he slipped on some ice, hit a tree stump and slid down the riverbank unconscious.
Live Action TV
Babylon 5: Out of jealousy, Lennier's final act on the show was to leave Sheridan behind a locked door, in a room being flooded with toxic gas. Subverted, in that, A.) he has a change of heart and goes back to correct the mistake, and B.) he returns to find others have arrived to save the day, and is forced to go on the run.
Breaking Bad: Walt watches Jesse's girlfriend, Jane choke to death on her own vomit (she'd shot up with heroin). Jane had earlier demanded Walt fork over some drug money and threatened to rat him out. Made worse in that Walt had inadvertently moved Jane on to her back when he tried to wake Jesse up, and thus indirectly caused her death as well as refusing to prevent it.
Farscape: On one occasion, Chiana has the villain of the week at gunpoint. By this point in the episode, he has killed two of her dear friends, including his own brother, and terrorizing the mining colony where they live. And yet, she can't bring herself to shoot him. Instead, she lets of a round which hits an acid pustule on the wall behind him, spraying him with deadly acid. Then she just walks away.
Law & Order: UK: In the episode "Samaritan", based on the original Law & Order episode "Manhood"note Which had more or less the same plot, but involved a gang of officers instead of just one., a homophobic policeman is discovered to have essentially killed his (gay) colleague by not getting him any help when he was shot (the courtroom section of the episode is mostly based around proving he was there and deliberately didn't do anything).
Luther: The biggest source of blackmail against Luther comes from the opening scene in the pilot, when a child molestor nearly falls to his death while fleeing capture. Instead of helping the molestor back on his feet, Luther lets the man fall to his death.
Medium: In one episode, a young Allison has visions about one of her friends. She sees that, by knocking on his door, she will stop him from killing himself, and many years later he will rape and murder teenage girls. So a few days later, she decides to not interrupt his suicide.
Midsomer Murders: There's one where a snobby wine lover is tied to his lawn while the murderer is catapulting wine bottles at him. His wife comes to the window and sees the whole thing (though the murderer remains unidentified). When she sees the bottle miss, she calls out corrections to the murderer. The next morning, the police arrive but she of course didn't see anything.
Nashville: Teddy watches as Lamar Wyatt has a heart attack. He begins to step forward to help, then stops, not even calling 911. Lamar dies.
Orphan Black: Suspecting Aynsley to be her monitor (erroneously), Alison does nothing to prevent Aynsley from accidentally strangling herself with a scarf and a drain grinder.
Person of Interest: At the end of the episode Reasonable Doubt, John decides the POI and her husband just aren't worth saving, and leaves a gun for the husband to even the odds in allowing them to kill each other.
In Carrie Underwood's "Blown Away" a young girl gets rid of her abusive father by locking herself in the cellar while he’s passed out drunk and there’s a tornado headed straight for the house.
A popular Urban Legend surrounds the Phil Collins song "In The Air Tonight". The legend usually involves someone watching someone else drown to death and being unwilling to help along with several other variations. In actuality, the song was about a divorce.
For Better or for Worse: In Michael's book Stone Season, the heroine suffers constant abuse and suffering at the hands of her cruel, controlling husband. In the climax, she goes out to search for him after he spends too long out on a ride, and finds him lying in the snow, injured. She simply turns around and heads home without him, leaving him to die.
In Lillian Hellman's 1939 play The Little Foxes (later made into a 1941 film starring Bette Davis), Horace decides to cut his evil wife Regina out of his will, and tells her so. Shortly thereafter he feels a heart attack coming on and asks Regina for his pills. She does nothing, instead watching as he collapses. He dies a few hours later without changing his will.
Little Shop of Horrors: Seymour tries to shoot Orin the Depraved Dentist, but can't bring himself to. Moments later, Orin gets himself high inside a mask full of gas, but finds he can't get it off and begs Seymour to help him get it off (while he laugh maniacally.) Seymour just stands by and Orin suffocates.
Amnesia: Justine: The titular character is presented with the option of doing this three times.
Ghost Recon: Future Soldier: The Ghost team deployed in Russia was about to take out the leader of the coup in Moscow when they were ordered by the US government to take him in alive. Ghost Leader is pissed off at this, but doesn't do anything to help the coup leader escape from being killed by an incoming train since he said "our orders were not to touch you."
Norman Jayden can do this if he fights the Origami Killer in the end. The alternative is to save him and then kill him.
Shelby can also do this to Charles Kramer by not giving him his pills while he's having a heart attack.
In Ghost Trick, when Sissel speaks to Lynne after she dies and tells her that he saw a video feed of her shooting him, Lynne comments that he could easily get back at her for it by simply leaving her dead. Sissel has no intention of doing that, however.
Dragon Ball Z Abridged: Following the scene where Krillan has mortally wounded Vegeta (as Vegeta requested), Dende is understandably unwilling to heal him on account of his partaking in the Namekian genocide. He reluctantly does so when Gohan points out how screwed they are without Vegeta's assistance.
Avatar: The Last Airbender: In a flashback showing parts of Roku's life, Sozin leaves Roku to die when the latter accidentally inhales toxic fumes from an erupting volcano.
In the season one finale, Zuko considers doing this to Admiral Zhao when the latter is attacked by the (giant, enraged) spirit of the oceans. After a moment's consideration, he holds out a hand to save Zhao, but the admiral refuses to allow his rival to save him, and is swept away to (presumably) his death.