In one of the arcs of Higurashi no Naku Koro ni , Satoko is killed like this when she is left as the sole survivor of Hinamizawa Disaster.
Gaara tries this with Rock Lee in Naruto, pre Heel Face Turn. He is stopped by Shikamaru and Naruto.
Of course, Lee didn't know any particular information. Gaara was just nuts enough to want him dead.
More specifically, Gaara couldn't comprehend why Gai would risk his own life to protect his protege, when Gaara's own mentor tried to kill him. Rather than try to confront the fact that other people have love and he doesn't, he decided to remove the reminder.
Kabuto attempts this with Sasuke. Then Kakashi gets in the way. And we still don't know exactly what Kabuto's motives were.
Happens to Rally in Gunsmith Cats. No outside assistance, and indeed she was never more than half awake as she shot the assassin down and threw the bomb under her pillow out the window.
This is done all the time! When nobody is able to kill the Major in combat, they try something else and send an assassin to kill her when she gets her cybernetic body replaced and is nothing more than a Brain in a Jar for a few minutes. Then the assassin looks away just long enough for the Laughing Man to plug her back in...
In the 14th album of the XIII series (Danger to the State) Irina Svetlanova disguised as a nurse attempts to do it to the main protagonist.
One Small Kindness: attempted upon Itachi by Kabuto, while he was recovering from his eye surgery.
Possibly Mikoto though so subtly that one might miss it until a second read-through. Mikoto was injured severly and comatose before she died, suddenly and without warning. The cause of death is apparently mundane, but she was a known political opponent (some would even say, rival) of Shimura Danzou. And, as mentioned, Kabuto was at the hospital...
Touched on in Dungeon Keeper Ami and featured following Ami's first real battle as a Keeper (commanding her forces, as opposed to adventuring alone like a hero). The battle against the Keeper Malleaus is a grueling contest of attrition, and leaves her utterly exhausted and lightly wounded. Upon her return to her own dungeon she collapses and has to be carried to bed. Jered, a heroic adventurer she met while exploring the surface world and recruited mostly due to convenience, briefly considers killing her in this fashion, due to the fact that she is at her most vulnerable. It is a true testament to how obvious Ami's heroic nature is that he doesn’t, every other instance of Keeper in the series is unapologetic evil.
In the Rocketeer, the german giant does this to one of mobsters who hid the jetpack, to prevent them from talking to the FBI.
Barely averted in both the first and second Godfather films. In the first Michael Corleone arrives to find the police guards have been 'reassigned'. He gets a nurse to move his father's bed to another room, then bluffs the killers by standing outside (with a friend) and sliding a hand inside his coat when the carload of killers turn up. The killers assume the guards are still in place and drive off. In the second film the hitman (this time working for Michael Corleone), having distracted the nurses with a cake, is about to press a pillow over the patient's face when a squad of Cuban soldiers, who have been tipped off about the hit, march in and shoot him.
Elle Driver disguises herself as a nurse and attempts to do this to The Bride in the first volume of Kill Bill. Fortunately Bill calls at the last minute and cancels the hit.
Bethany from Dogma saves "existence" by doing this.
Similar thing happens in Hard Boiled, only that the victim is not unconscious but rather, in a plaster cast from tip to toe, which only makes the murder (with a scalpel) crueler.
The Steve McQueen thriller Bullitt (1968). A hitman sneaks into the hospital with the intent of finishing off his target, but is recognised and chased off by Bullitt.
Telefon. A priest who's been brainwashed to commit sabotage is captured alive. A female KGB agent purchases a nurses' uniform from an on-site outfitters shop, while Charles Bronson (posing as a doctor) rings up the duty nurse and tells her that another nurse will be replacing her, as she's to report somewhere else. The false nurse then injects air into the IV tube of the patient.
The Dark Knight: The Joker dresses up as a nurse (with hella scary lipstick) to visit a badly injured Harvey Dent in the hospital. He doesn't kill him, but his visit is enough to send Dent over the edge and make him fully embrace his new nature as the vengeful Two-Face. The Joker does, however, kill the two cops standing guard outside Dent's room.
The Dark Knight Rises: A few of Bane's henchmen break in to Gotham General Hospital where Gordon is recovering from gunshot wounds Bane gave him in the sewers, with the intent to kill him. This being CommissionnerGordon, he manages to turn the situation to his advantage and gets the jump on three guys, and almost gets Blake.
Dean Koontz's The Door To December features a hitman sitting in his car, preparing to sneak into a hospital in the middle of the night to whack a certain patient. The next time we see him, he's still sitting in his car — with his neck crushed by an unknown assailant.
Happens very early in A Song of Ice and Fire to Bran Stark after he witnessed Twin Cest between the queen and her brother Jaime Lannister. Subverted, when it turns out Joffrey arranged it as a Mercy Kill. He thought it was; nobody else did, and the act triggers a war between the Starks and the Lannisters.
Able Team ("Justice by Fire"). A corrupt FBI agent goes to interview the sole survivor of an Able Team ambush. It turns out he's willing to tell everything about those paying the FBI agent, who notices that the food service workers in the hospital don't have any form of identification. A few hours later, a man dressed as a food service worker presses a pillow over the patient's face.
In Bored of the Rings, after the battle of Minas Troney, Farashlax is assigned a private hospital room, where he is visited by an apron-and-stethoscope-wearing Arrowroot (already now claiming the title of "true king of all Twodor"). The Ranger emerges with his apron covered in blood, and explains that Farashlax succumbed to "terminal abrasions and contusions with complications," though Legolam didn't think he was wounded that much.
In The Remittance Kid by J.T. Edson, one of the anarchists sneaks into a hospital disguised as a priest and uses a pillow to smother a wounded accomplice before he can talk to the police.
The Forever War. A soldier who tries to frag Major Mandella dies on the operating table (after he attempted suicide) under the care of a doctor who has a covert attraction to Mandella. Unfortunately the suspicious timing of this death offsets the morale problems Mandella avoided by not having the soldier Thrown Out The Airlock.
This is invoked in season eight — CTU release a news bulletin saying that the very dead Farhad Hassan is alive and has been taken to hospital in the hope that the villain will send one of his men to finish him off, who they can then capture.
Later in season eight, the wounded Samir Mehran is killed by an assassin disguised as an EMT as he's being loaded into an ambulance.
In an episode of Family Matters, Carl ends up in a hospital after being hit with a bullet during a shootout. A shooter, seeking revenge for his relative who Carl recently arrested, dresses up as a doctor and sneaks into the hospital room intent on killing Carl. Steve sneaks up behind the shooter and knocks him out with the metal squatting pan.
An unusual humorous (if morbidly so) example occurs in Rescue Me, in which a character suffering from kidney cancer awakens from a musical Dream Sequence gone awry to find a well-meaning uncle attempting to smother him as an act of mercy because the doctors say he's taken a turn for the worse.
Leverage, "The Beantown Bailout Job." The bank manager behind the whole scam is on his way to the hospital to kill the only witness, who's lying in a coma... only to find the Massachusetts State Police waiting for him when he arrives.
The X-Files. After A.D. Skinner is shot Scully thinks this has happened to him when she finds his room empty and the police guards missing. It turns out he's being transferred to another hospital. She decides to accompany Skinner there in the ambulance, which is just as well as that's where the attempted murder occurs.
And Skinner himself attempts to do this to the comatose Mulder in "Deadalive", thinking he is saving Scully's unborn child by doing so. It fails, but ends up saving Mulder from a life of alien replication.
Castle. In season one, this almost happens to a man in witness protection - luckily, it turns out just to be a trap, and our heroes catch the would-be assassin.
In The Pretender Miss Parker is sent to kill Sydney's twin Jacob, who was in a persistent vegetative state after a car accident caused by him wanting to leave The Centre. She faked his murder with Sydney's help. In a later episode this was found out, which lead to sweepers were sent in to do the job properly. They did not arrive on time.
In Boardwalk Empire, one of the gangsters attacked in the first episode turns out not to have been killed by Jimmy and Al and turns up badly wounded. Since Nucky is afraid he will start talking and name names, he arranges things so that his brother will smother the guy with a pillow while pretending to question him. This plan is interrupted by the FBI, but things don't go better for the patient after that, as the Knight Templar Agent Van Allen tortures him for information via Open Heart Dentistry, and he dies after giving up information.
In episode 6 of The Shadow Line, this almost happens to Gatehouse when he's in hospital after being shot by Glickman. However, he's able to overpower and kill his assassin and escape.
Attempted on Angel, though for revenge rather than to silence someone.
Luther: sociopathic killer Alice Morgan disguises herself as a doctor, hits the fire alarm and then tells the police officer guarding the man she intends to kill that there's some violent patients on the floor below who are hindering the evacuation. She then walks into the room and smothers her target with her bare hands.
On Seinfeld, George is in the hospital and keeps going on about how he doesn't want to suffer and just be sick forever so he asks Jerry, that if it comes to that, to put him out of his misery. Jerry says "Like this?" grabs a pillow and start suffocating him while he struggles.
In the 2010 TV version of Macbeth starring Patrick Stewart, which moves the location to a 20th century dictatorship, the three witches are disguised as hospital nurses. They're first seen "taking care" of the wounded soldier who's just brought news of Macbeth's victory to the king.
Happens in the Dark Angel 2nd season episode "Harbor Lights". While Max was in the hospital, a nurse (who was actually an agent of an evil organization) tried to kill her with drugs.
Lampshaded in Game Of Thrones when a Genre Savvy Tyrion wakes up after nearly being killed in a Bodyguard Betrayal to find Maester Pycelle (whom he had thrown into the black cells in an earlier episode) smirking down at him.
Tyrion:POD! (Tyrion's loyal squire rushes in) Find Bronn, or Varys. Tell them I am here with Maester Pycelle. Tell them, I am very much alive.
Deep Space Nine. A killer intending to finish off Quark visits him with a bunch of flowers, which he shoves against the Bajoran deputy's chest, then removes to reveal he's stabbed the man with a knife. He then uses a Vorpal Pillow to finish him off, only to be interrupted by Quark's brother who starts screaming; Odo rushes in with more deputies and arrests the killer.
Odo: It's over Rom, you're a hero! You saved your brother's life.
(Rom starts to smile, then realises he's going to lose the bar if Quark lives and starts screaming again. On his not-so-deathbed, Quark smiles.)
On The Borgias, Cardinal Sforza is enlisted by his family to do this to Rodrigo after the latter survives being poisoned, but he's interrupted before he gets the chance and so far he's ended up siding with the Borgias after all
In the playtesting of the first adventure "The Dreamer", the PCs entered Paul LeMond's hospital room to find the cultist Clarence Rodgers strangling him. Rogers had infiltrated the hospital dressed as an orderly.
The Worm That Walks. While a PC is in the hospital Mr Edwin sends the title monster to murder him by ripping him to shreds. The monster will masquerade as one of the PC's colleagues until it can attack him.
In the end of Dreamfall, Zoe Castillo is drugged by Helena Chang to save the real world via dreaming. After she is done, Helena injects her with even more drug, to make sure she never wakes up again. Apparently, it only works half-way: Zoe cannot wake up but possesses a lot of power even when sleeping.
Considered in the first Persona game by Nanjo at upon discovering that their friend Maki is trapped in Kandori's DVA system, which has been the cause of all their problems. Mark decks him for it.
In Sailor Moon Another Story, a youma tries killing Hotaru while she's overcome with dark energy. Haruka, being the only character cynical enough to prepare for such an eventuality, has, of course, prepared.
In Broken Sword you must pretend to be a doctor to speak to someone, however an assassin does exactly the same thing, invoking this trope. Nico lampshades this.
Apollo Justice Ace Attorney has the killing of Magnifi who was dying in hospital from liver cancer and diabetes when his murderer apparently came in and shot through the head. Subverted in that he ordered his own killing. And further subverted in that neither of the people who he ordered to kill him could bring themselves to do it, and so he committed suicide.
The reason Gil from Girl Genius had to send out Captain Vole to try to get Agatha is because waves of assassins were trying to kill his hospitalized father. The first assassin plays it cool slightly better than most when questioned. Slightly.