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In Mirai Nikki, it's shown very clearly that Yuno Gasai (pictured above) would be all too willing to kill ANYONE who got in the way of her "love" with Yukiteru (including his mother and father). In fact, Yuno is even willing to kill anyone who is less than ten feet away from Yukiteru simply because they are less than ten feet away from him. Heck, she's even willing to kill Yukiteru AND HERSELF just to be with Yukiteru. Essentially, if she doesn't kill him the world will end and they will both die. If she does kill him then she can go to a different world's timelime, kill that timelime's version of herself, and then be with that timeline's Yukiteru. She doesn't seem to really mind that each of the world's Yukiteru are technically different people since her "love" for him is just to satisfy herself, at least until the end of the story, and she herself admits this.
A less lethal version of this seems to be executed by Aru Akise, who has a crush on Yukiteru. He goes through the trouble of researching everything about Yuno's background to try to dig up dirt on her, and exposes it to Yukiteru, telling Yukiteru to "get away from her." Chapter 50 seems to have confirmed this. When Yuno shows up, Akise glares at her, declares "I won't hand over Yukiteru to you!", and forcefully kisses Yukiteru. This sends Yuno into a rage and she comes at Akise with a knife. Akise tries to kill Yuno, because he believes she's trying to kill Yukiteru, and destroy her diary but ends up failing, because it turns out Yuno had two diaries, and she ends up stabbing him. In the following chapter despite having just been stabbed by Yuno and sliced across the throat, Akise gets back up out of sheer determination (and love for Yukiteru) and texts an important message to Yukiteru even though Yuno ends up cutting off his head completely.
The ending to the School Days anime, where Kotonoha violently Murders The Hypotenuse, right after the Hypotenuse, Sekai, murders the lead, Makoto, thus killing the triangle.
This was reversed in the manga, where Kotonoha is the Hypotenuse that ends up killed after trying to kill Sekai.
This is often the path villains take in Sailor Moon. Beryl, Ann, and Black Lady try to kill Usagi to get Mamoru, while Al and Diamond try to kill Mamoru to get Usagi. Emerald also tries to do in Usagi to win Diamond.
Fiore in the R movie tries to kill Usagi both because she is Sailor Moon and out of jealousy over Mamoru, whom he is attached to because of how kind he was to him when they were kids. He gives up on it by the end and even sacrifices his Life Energy to revive Usagi when she dies saving the earth.
Also invoked by Al and Ann from the Makaiju Filler Arc, especially in the end when Al orders the tree to kill Mamoru so he can have Usagi, then Ann retorts by ordering it to kill Usagi so she can have Mamoru. This sends the Makaiju into a Villainous Breakdown where it goes Kill 'em All. It all gets better in the end, though, and Al and Ann leave Earth together and free of their Yandereness.
In Haruhi Suzumiya, Haruhi is willing to destroy and rebuild the entire UNIVERSE again with only her and Kyon if anyone gets too close to Kyon and she becomes jealous. Mikuru's closeness to him along with her boredom caused the first incident... without Haruhi even noticing that she had done so.
Sae from Peach Girl hates Misao for being the one Ryo loves since she loves him herself and, at one point, tried to push her into the path of a moving bus. However, Kairi restrained her at the last second so she wasn't able to.
Kairi's trio of Clingy Jealous Girls absolutely hate Momo for winning Kairi's affections. They make a habit out of harassing/threatening her, once deliberately made sure she got separated from Kairi and was stranded during a class trip, push her into the path of a camp fire when she was holding cooking oil, and, at one point, held her down, and told her that she has to sign a contract swearing that she'll stay away from Kairi or they'll burn all her hair off (while holding a lighter to her hair), and try to beat her up later on when she states that their selfish and cruel actions won't make Kairi fall in love with them, and when she tells them that her love for Kairi is genuine. In addition, they take the threat even further with "We'll burn your face so badly that Kairi won't ever look at you again," and "Don't worry, we'll push you into the pool before your whole body catches fire."
Peacemaker Kurogane: In the manga, Suzu comes to this conclusion when Ryoma decides to take Tetsunosuke away from him.
Code Geass: Rolo does this to Shirley, Lelouch's "girlfriend", not because of her connection to Lelouch, but because he wants to kill Nunnally and stop her from coming between him and Lelouch. Shirley was an obstacle in making that happen, as she told Rolo that she wanted to reunite Lelouch and Nunnally and thus she had to go.
Also, Mao's treatment of Lelouch seems this way at first - C.C. originally made a contract with him, but abandoned Mao and made one with Lelouch instead. So Mao tries to kill him by manipulating a girl who has unrequited love for Lelouch, turning into a temporary Yandere. It fails. Somewhat averted in that Mao offers to leave Lelouch alone if C.C. comes with him. Concerned with losing one of his best allies, however, Lelouch prevents this. After that, Mao directly goes after Lelouch in the most potentially destructive manner possible.
Also V.V., who looks like a kid but is Really 700 Years Old, became psychotically devoted to his younger twin brother Charles after their mother's death and killed his sister-in-law Marianne, Charles's favorite wife and his Knight, out of jealousy when they started getting too close.
Tatewaki Kuno: Has never actually tried to kill Ranma for Akane or "the pigtailed girl's" hand, and doesn't even seem to want to kill Ranma in their fights. His attitude of "if I beat up Ranma, then the girls I want will naturally flock to me for proving my studliness" does fit under the general umbrella of this trope, but lethal force is not his intention. Though... there is the way he starts swinging around a live steel katana instead of his bokken during the Tanabata and Failed Wedding stories, which probably inspired his fanon reputation.
Ryoga Hibiki: Has no connection with this trope at all. While it is argued over whether or not he did actually want to kill Ranma during his first story, even if he did, the cause was over Ryoga's curse, not Akane Tendo. Any murderous intentions Ryoga might have had were gone by his second story, anyway. Whenever Ryoga attacks Ranma because of Akane in all subsequent stories, the general cause is "protect Akane" or "punish Ranma for making Akane angry/sad". The one time Ryoga did try to kill Ranma was when he accidentally caused Ranma to fall in love with him, which made Ranma act in the same way that Ranma's own "openly affectionate" fiancees act towards Ranma. Ryoga did admit to himself that Ranma being dead would leave Akane "free" to his courtships, and had an Imagine Spot while digging Ranma's grave about Akane confessing her love for him on that very spot, but his initial and primary motive was purely disgust with/fear of Ranma and Akane's "falling for him afterward" provided additional motivation. However, at one point he did plan, together with Mousse, to accompany Ranma on a dangerous mission against a quite murderous villain just to make sure he killed their rival in the process (and conveniently seal away their curses while they were at it.)
Kodachi Kuno: Joined in with Ukyo and Shampoo in attacking Nabiki during the "Nabiki, Ranma's New Fiancee" manga arc, in hopes of killing her and taking him for herself. Similarly, teamed up with them to beat up Miss Hinako during the manga arc she was introduced in, as she suspected the teacher was trying to seduce Ranma. She did show at the Failed Wedding in a wedding gown of her own and planning to marry Ranma herself, but wasn't slinging bombs around like Ukyo and Shampoo.
Ukyo Kuonji: Generally prefers more "nonviolent" approaches to trying to win Ranma, such as setting Akane up on a date with Ryoga, trying to lure Ranma & Akane into a magical Kimodameshi that will supposedly break them up irrevocably, or agreeing to let Ranma stay at her home after he is kicked out (and, later, to let him use her business as a wager on the condition he will join her in resuming her travels if he loses it) in hopes that this will make him "realise how perfect she is for him". However, she can go about it violently as well; she willingly joins in with Shampoo and Kodachi when they attack Nabiki with plans to kill her and then fight over who gets Ranma, and is the one who suggests Hinako might be trying to seduce Ranma and leads the other two girls in ambushes on her. She shows up at the Failed Wedding throwing bombs, though possibly at Ranma rather than Akane, and wonders why she didn't think of it herself when Kodachi reveals her plan to use this ceremony as a way to marry Ranma at last. In the anime, though the aforementioned attacks on Nabiki and Hinako are removed, Ukyo plots to herself about killing Akane when she asks for her help in the second of the "One to Carry On" fillerOAVs and begins throwing random heavy objects at her, growing angry when Shampoo throws something at her and leaving a clear impression she thought that she and Shampoo would work together to kill Akane, and then finish fighting each other.
Shampoo: Given she stated "obstacles are for killing" when Ranma tried to prevent her attacking Akane for Ranma's hand in her first story, it's understandable that she gets hit with this trope the most — in canon and in fanon. Fanon usually overstates her actually trying to kill her rivals, though. Canonically, while the idea doesn't seem to bother her in the slightest, it isn't something she tries all of the time. Notably, when she first thought Akane was an obstacle to pursuing Ranma's hand, she actually just wiped her memories of Ranma. Then, when it looked like Akane might be breaking through the memory block, she tried to strengthen the block, only attacking after Akane did break through the block and kept interfering — and even then, she stopped attacking Akane the moment Ranma told her not to. After that, though, she tried to give Akane a Jusenkyo Duck curse in order to "pawn" Mousse off on her, plotted to murder Akane and pin the blame on Taro in the first Pantyhose Taro story, was the first to suggest attacking Nabiki during her stint as Ranma's fiancee in the manga, joined in Ukyo's plot to beat up Hinako in the manga, came up with the same plan as Ukyo in "The One To Carry On, Part 2" and wrecked the plan by attacking Ukyo first instead of Akane... the final manga story arcs, Jusendo and the Failed Wedding, had a Brainwashed Shampoo (who, given she was obediently sent to murder Ranma, even if she did muster some Villainous Willpower to try and wriggle out of obeying, was arguably Brainwashed and Crazy) threaten to pop off the dollified Akane's head and a normal Shampoo attacked Akane & Ranma with bombs at the wedding ceremony.
Mousse: Introduced to the series specifically challenging Ranma for the right to Shampoo's hand, threatening to take Akane if Ranma couldn't beat him. On his return, attempted to murder Ranma by using him/her as the target in a knife-throwing act (Ranma managed to catch the knives in her teeth) and then chased him around the circus throwing daggers and water balloons full of Jusenkyo Duck water at him. Shortly after that, he kidnapped Akane and threatened to douse her with said Jusenkyo water, in reality planning to either murder or curse (whichever came first) Ranma when he raced to Akane's rescue. He then challenged Ranma to yet another duel to win a date with Shampoo. Repeatedly throughout the series, Mousse challenges Ranma to fights because, like Kuno, he thinks that beating Ranma will make Shampoo fall in love with him — unlike Kuno, Mousse doesn't seem bothered by the idea of killing Ranma if that's what it takes to win, though he has never actually "won" except in fights where he had other things on his mind and so passed over the opportunity to deliver the coup de grace.
On the topic of Kaishaku's works, an interesting case of this occurs in Shattered Angels where resident Psycho Lesbian Mika Ayanokoji tries to sacrifice the hypotenuse. Ultimately it's cut short before anyone's sacrificed, but is ironically turned around when Mika is killed off rather nicely.
Kish from Tokyo Mew Mew is a stalker who constantly attempts to get rid of Ichigo's boyfriend Masaya. He even attempts to kill Ichigo herself if she refuses to become "his".
A mildly-popular theory that was ultimately jossed by canon invoked this, via speculating that Billy Katagiri had caused the incident that led to the death of Emilio Ribisi, all to get close to Emilio's girlfriend Leesa aka Sumeragi. In truth, it was Sumeragi and Kati's own mistakes that brought said disaster; Billy did later show Yandere tendencies for her, but in this he had nothing to do.
And in Mobile Suit Gundam ZZ there's Beecha, who plotted with Mondo to have Judau killed by unknowingly using real bombs that the two lied were fake. This is due to Beecha's jealousy over Elle swooning over Judau and the feeling that he needed Judau out of the picture to make himself look like a better match for Elle. Fortunately, it fails.
A non-romantic (we really hope) version also occurs, with Puru briefly attempting to murder Leina Ashta, because she wants to be Judau's little sister and is well-aware that Judau cares for Leina more than he does her. Judau manages to talk her out of it by promising that all three of them could be a family together after the war was over.
A Zig Zagged Trope in Hell Teacher Nube. Yukime was willing to kill her love rival, Ritsuko, but only after a very cruel Breaking Lecture from her father's lackey Ippon Datara, in which he first hit all of the girl's weak points and then told her that, if she wanted to become human and be able to be with Nube, she'd have to kill Ritsuko. So Yukime kidnaps Ritsuko and restrains her, apologizing all the way through said murder attempt... but she then decides that no, she can't kill someone who has done nothing wrong to her, and lets go of her. Which ends up with Yukime dying as she's tortured to death by Datara. (She gets better, however).
In Mai-HiME, Shiho Munakata is in love with her childhood friend Yuuichi Tate, who's in love with Mai Tokiha. When Shiho comes to believe that Mai is stealing Tate away from her (more due to Tate's actions than Mai's), she attacks Mai to keep her out of the way permanently, which isn't helped by Nagi preying on Shiho's insecurities later. The true tragedy is that, as Mai realizes in the fight, due to the mechanics of the whole Himelander thing going on, if either of them loses the fight (since Mai realizes she is beginning to develop feelings for him, as well), Tate will die. This doesn't stop Shiho, though, as she's gone over the deep end already, and Tate finally dies after Mikoto kills Shiho's Child.
Psycho Lesbian Tomoe from Mai-Otome wants Shizuru, and is perfectly willing to harm or kill anyone whom she perceives as a threat to her ultimate goal. This leads her to do nasty things to Arika, whose fangirl crush is seen as stealing away from any attention that might otherwise have gone to Tomoe. On top of that, Arika isn't even a real contender in this Love Triangle - that honor goes to Natsuki, Shizuru's boss.
Played with in Speed Grapher. Dark Action Girl Ginza is desperately in love with her friend Saiga, admits to be really jealous of his protegée Kagura (and once returns her to her mother, the one Saiga is trying to free her from) and does admit out loud that she'd love to kill her... but in the end, she decides to protect Kasgura with her life, since Saiga made her promise that she'd protect Kagura.
In the Rumiko Takahashi one-shot Laughing Target, the protagonist has a cousin to whom he made a Childhood Marriage Promise. However, he eventually grew up, moved on, and started dating a girl at his school. When the cousin moves in with the protagonist (after her mother's mysterious death), it turns out that she hasn't moved on - and she's very unhappy with the new girlfriend for taking what's rightfully hers...
The whole plot of Area 88 is that Shin's (now former) best friend ships him off to a war in the hopes that he'll get killed, so he can marry Shin's girlfriend, the daughter of his company's CEO.
Creed from Black Cat does exactly this to Saya when Train started developing a deep friendship with her. When that didn't manage to convince Train to come to him, Creed decides that it is now Train's Heterosexual Life-Partners fault and tries to kill him. He tries to do this to Sven and Eve later in the series.
Genkaku from Deadman Wonderland attempts this twice with the girls he thinks Nagi is interested in. He stabs Karako when she gives Nagi a Cooldown Hug, calling her a "shitty girl," and telling her not to touch Nagi. He also is shown to be very agitated and jealous when he insinuates that Shiro is someone close to Nagi, saying that Nagi "never learns," and that he'll just have to punish him again (which he tries by attempting to have Shiro and Karako raped). It's especially interesting to note that he was also the one that killed Nagi's wife in the beginning.
Masaru "Chaplin" Sukegawa was imprisoned after murdering a woman that her lover was caught with. Though she didn't specifically aim to murder that person, she still did it since her Branc of Sin manifested there.
Shiro did this in the first chapter/episode. She slaughtered Ganta's entire class out of jealousy, and then proceeded to show Ganta Mimi's severed head.
Full Metal Panic!. When it comes down to it, this is pretty much what Gauron tries to do to Kaname, because he's angry that Sousuke fell in love with her.
A variant of sorts occurs in Fist of the North Star in regards to Shin, Yuria and main hero Kenshiro. Shin is in love with Yuria, who is in a relationship with Ken. The variant here though is that Shin seems relatively self-restrained... until Jagi, Ken's arsehole brother, comes along and convinces Shin that Ken is too weak to protect Yuria and that Yuria would end up dead if she stayed with Ken any longer. Result? Shin and Ken fight, Ken loses, Yuria declares her false love for Shin to spare Ken's life and he takes her to his Southern Cross kingdom and some time later, Ken confronts him there. Fans know how the story goes from there.
Rukia's adoptive younger sister Homura, and to another extent her adoptive younger brother Shizuku, from the third movie wanted to kill all the Shinigami to keep them from coming between them and Rukia.
Orihime's hollowfied brother Sora also qualifies, trying to kill Ichigo (and Tatsuki, though less so) for taking his place in Orihime's heart.
Angel Sanctuary: That's pretty much the motivation for Rosiel's schemes against Sara. Alexiel won't wake up permanently (shows up in moments of need but always gives back the control to Setsuna) because her current reincarnation is extremely happier. So Rosiel figures that if Setsuna could not be happy anymore... And yes, Rosiel and Alexiel are brother and sister, but that barely matters to the point of this trope. And even if it did, Sara and Setsuna are too, but that didn't keep her from being overjoyed when she believed to be carrying his child.
And on a straighter example, Layla set up her (successful) love rival to be killed by the man they both loved. Pretty cruel for all of the involved.
And while Raphael doesn't actually try to kill Setsuna, he does make it very clear he'd be glad to step in should Setsuna die.
Suruga from Bakemonogatari is infatuated with Hitagi. So much so that she starts stalking Koyomi after he hooks up with Hitagi and tries to beat him to death twice. Doesn't manage only because of Koyomi's amazing regenerating ability. I bet having been thrown around the room and then pulled back by your intestines would kill a normal person
Sort-of subverted in InuYasha, when Koga first decides (completely unilaterally and over the course of about five seconds) that Kagome is "my woman." Upon realizing that Inuyasha is possibly in the way, Koga cheerfully proclaims that he will simply have Kagome by killing Inuyasha. He proclaims this to Kagome. He's blissfully unaware that she might have a problem with the idea. (He doesn't actually do it, of course, or even really try. But the logic is the same).
Played straighter by Onigumo/Naraku towards Inu Yasha re: Kikyou. It's a little iffy due to the majorly conflicting motivations between his human and youkai sides, which hate each other as well.
Kikyou once tries to murder Kagome because she is "in her way", and attempts to drag Inuyasha to Hell with her. She loosens up as her rage at the world disminishes and then she focuses on destroying Naraku for once and all.
Get Backers: Takuma Fudou to Ginji, over Ban. "When you're around, Midou doesn't even notice me. Midou's eyes are not on me... because you're in the way!"
Kazuki's gang, Fuuga (lit. "Elegance"), is really into the Ho Yay. There's some kind of Toshiki/Kazuki/Juubei Love Triangle, apparently; Toshiki even attempts to Murder the Hypotenuse, claiming that Kazuki never paid any attention to him when Juubei was around.
Jegan fakes this in Rave Master. He kills everyone but the other two members of the love triangle (since he happens to be friends with the guy his crush liked more) then convinces each of him that he killed the other.
Outlaw Star: Harry attempts to get rid of Gene in episode 21, once he realizes Melifina was in love with him. And tried to do the same to Hazanko for coming between them at the Galactic Leyline. Neither attempt ends well for Harry, especially the latter.
Baka to Test to Shoukanjuu: Miharu's reaction to seeing anyone trying to come between her and Minami (who is not even interested in her) is to chase that person down and try to kill them.
Mizore in Rosario + Vampire tries to kill the rest of Tsukune's harem and turn Tsukune himself into a popsicle so she can have him all for herself. However, once she's added to the harem she becomes friends with her love rivals and continues to stalk Tsukune. She says herself that she doesn't really "need" to stalk him anymore, it's just "more fun."
In chapter 52 of manga season II Akuha is revealed to be a Yanderefor Moka. She attacks Kurumu and Mizore saying that she will kill anyone who tries to take Moka from her.
In chapter 53 she announces her intentions to kill Tsukune.
Neon Genesis Evangelion: Naoko Akagi was sexually involved with Gendo Ikari after the death of his wife Yui Ikari, though it is suggested that Naoko wanted Gendo before, as she described Yui’s disappearance as being exactly what she’d hoped for. Naoko still strangled the first Rei Ayanami when the child said that Gendo considered Naoko as an "old hag" and no longer useful (Rei’s resemblance to Yui might also have been a factor, as Yui’s face appears superimposed over Rei 1’s as she speaks to Naoko). She was later horrified that she had done so and committed suicide.
Also, Ritsuko Akagi was romantically involved with Gendo, as her mother Naoko was before her. However, she eventually realizes that Gendo is using her in the same way that he used her mother, and that he cares about Rei more than her, so she likens Rei to that of a romantic rival. In the manga her disdain of Rei is more evident: at one point, during one of Rei's medical checkups, Ritsuko loses control after Rei rebukes her and tries to strangle Rei to death. Realizing that she was acting like her mother before her, she stops herself from killing Rei, but warns Rei against telling anyone what happened. She also destroys the Rei clones in a bid to get at Rei and Gendou.
Sakura Gari: Sakurako and Katsuragi both try to do something of the sort to Masataka in order to have Souma for themselves. And Sakurako spends a good amount of her time trying to drive away any person that Souma gets involved with via torturing them into insanity, i.e, she causes a pregnant servant girl to lose the baby of Souma's that she was pregnant with. (Or in the case of the artist Terashima, breaking his drawing hand and then pushing him down a flight of stairs.). And some of the people that Souma took as lovers in the past tried to do this when he tried to get them to leave him alone. One of them, Yoshino, tried to stab Souma in the streets, and when refused he was Spurned Into Suicide.
Onii-Chan Control features Noa, the little sister to the Main Character, who revealed she's willing to stab a girl she thought was interested in her brother. Of course, she never shows that side of her whenever her brother is around.
Pandora Hearts: Gil's devotion to Oz has a pretty dark edge. Hinted early in; outright shown in chapter 33, where Vincent goads him into saying/resolving to kill anyone who hurts or snatches his master away from him - no matter who it is. He almost went through with this concerning Alice. Luckily, he stops himself in time and thankfully, Alice didn't notice a thing.
Maki in Darker Than Black has a massive, crazy stalker-crush on his boss, Amber. Such an insane crush, in fact, that he completely abandons the usual Contractor "emotionless" act when he finds the guy Amber's in love with, and tries to kill him even though if he'd succeeded, it would have ruined Amber's entire plan, resulting in a genocide.
Sayaka Kouno of Princess Princess once shoved one of Tooru's previous girlfriends down a flight of steps out of jealousy. And then there's her habit of stabbing Tohru's girlfriends with scissors.
Mirage of Blaze: Naoe does this by raping Kagetora's lover and later on forcing Kagetora to possess her to save his life.
Ai no Kusabi has Guy, who is so in love with Riki, not only does he give him a penectomy to remove the pet ring that was on his penis upon being told that as long as Riki wears Iason's pet ring he'll never be free of him, but he also blows up an entire fortress in an attempt to kill Iason. It backfires horribly when Iason is fatally injured, but Riki actually chooses to die with him.
Both Mika and Namie of Durarara!! are Yandere for Seiji who's pretty crazy himself seeing as how he's in love with Celty's head. Mika has...plans for Celty's real head if she ever gets her hands on it and Namie actually tried to destroy it.
Professor Nasujima at Mikado's school clearly has a thing for Anri, which naturally Squicks the hell out of her. The same guy is also dating Haruna Niekawa (one of his former students). Or was. Haruna doesn't like that he's changed his preferences, so she gave Anri a visit and tried to kill her...
In the yaoi manga Under Grand Hotel, in the first few scenes we see Lain Brody in he gives Sen a blowjob, strips him, drugs him and outright molests him as he's drugged (including telling him "No way in hell am I letting Sword Fish kill you. Because he doesn't love you. I'm the only one that can kill you."), rapes him with a mop handle, ties him up and hides him in a dryer. And when Swordfish finds Sen he gives him CPR. Lain responds to this by stabbing Fish in the arm with a fork and tries to stab him again while screaming, "Get off of Sen! He belongs to me!" Immediately after that Lain is shot and killed by security guards.
Also, Swordfish threatens anyone who expresses an interest in Sen with death, kills the guys who raped Sen saying that he did it for Sen's sake even after Sen told him not to kill them, moves out of Sen's room and lets a rapist move in when angry with Sen, and slices the throat of another one of Sen's rapists right in front of him while saying "I love you, Sen." This being after he tried to get Sen to kill the guy himself but Sen refused.
And Norman's in love with Swordfish but pretended to be in love with Sen so he could get closer to Swordfish. He tried to persuade Swordfish to kill Sen but that didn't work. So, on one occasion when there was a scuffle going on he tried to get Sen out of the way by stabbing him with a pencil but accidentally ends up stabbing his inmate Walter instead who was trying to protect Sen in the hope of getting protection from Swordfish. When he finds this out, Norman holds a knife to Walter's neck telling him, "If you don't want to die, don't butt in again Walter!"
Hunter × Hunter: Shaiapouf is afraid Komugi might distract the King from his "true goal" and is therefore considering killing her for "the King's sake".
Clover: A thought B seriously got in the way of his romance with C/Ran. He also tried this on Gingetsu, but failed.
Ask Dr. Rin!: Tokiwa tries this over and over for four episodes straight in the hope of having Meirin for himself. It's eventually revealed he was possessed.
Basilisk: Kagerou is dead set on killing Oboro so she can have Gennosuke. (Much more in the anime than the manga, though.) Tenzen also wants to kill Gennosuke so he can have Oboro as his puppet wife, which would grant him full control of the Iga Tsubagakure.
In Katekyo Hitman Reborn!Lethal Chef Bianchi goes into a homicidal rage whenever she sees adult Lambo who looks exactly like her deceased ex-boyfriend Romeo, who apparently died of food poisoning. She also has an unrequited crush on Reborn and is perfectly willing to eliminate anybody who has a 'bad influence' on him.
In chapter 271 M.M. threatens to do this to Chrome. Chrome is on her way to deliver something to Mukuro and she encounters M.M. As soon as M.M. glances at Chrome she grabs her and yells at her before hitting her so hard that she falls to the ground, her mouth and nose bleeding. She then keeps calling her an "ugly bitch" and tells her: "The Mukuro-chan of this generation is mine! If you put your nasty little hands on him, I'll kill you!"
Liang Qi of Canaan appears calm and even-tempered, but is in fact unwavering, merciless, and very much obsessed with admiring and idolizing her Sister" Alphard, to the point of being in love with her. To her frustration, however, Alphard does not reciprocate her feelings and instead ignores or even proceeds to ridicule her from time to time. Because Alphard seems to have taken a special interest in observing Canaan's reactions to every plot carried out by Snake, Liang Qi sees the elimination of Canaan as a way to win back Alphard's attention.
In Xam'd: Lost Memories, Furuichi (in his Xam'd form) tries to murder Akiyuki in order to have Haru to himself. He fails, and the episode ends with Akiyuki getting Laser-Guided Amnesia after getting confronted by Commander Kakisu, Haru arrested for her relations to the Xam'd incident, and Furuichi beheading myself after finding no reason to live.
In episode 11 of Goshuushou-sama Ninomiya-kun when told Mayu might die soon (she's a succubus so she needs life force) Reika's alternate personality tells Shungo she doesn't care since "it's more convenient for me if Mayu dies like this, because then I can have you all to myself." She then agrees to let Mayu live so long as Shungo "becomes hers forever." This is also played with in regards to Shinobu, a maid of Reika's, who thinks Shungo is not worthy of Reika, and is always trying to "execute" him.
SHUFFLE!: Kaede snaps in episode 19 upon seeing Rin bring home Asa, whom he's dating. She screams at Asa to get out, and viciously attacks her. However, Rin restrains her and she struggles against his hold, screaming at Asa to "DIE!" Primula also discovers Kaede had crossed out Asa's face in some pictures of Rin and Asa together. She gets better, though.
Elfen Lied: Lucy tries to suppress her instinctive desire to kill all humans so she can live happily with Kouta, until she finds out he lied about the sex of the cousin he was going to a festival with and she saw them hugging, at which point she stops trying to be nice. She then proceeds to slaughter some people at the festival and kills Kouta's father and sister, and starts to go after Kouta's cousin Yuka, voicing her intent to kill her... but only stops once Kohta tackles her and she sees his tear-streaked face. She also isn't above acts of jealousy, once using her vectors to shove Yuka down when she saw her holding hands with him. Yes, she's VERY broken, but how broken she is is absolutely NOT an excuse for her killing or trying to kill innocent people just because Kouta pays more attention to them.
Gopher of Soul Eater beats Kid to a bloody pulp twice for having Noah's attention and got extremely pissed off when Maka turned out to have the same uniqueness that made Noah have apparently fleeting interest in Gopher. This gets VERY bad when Noah decides that he wants to have Maka as well. It doesn't get any better. With his chance of killing Maka gone, Gopher beats up Kid some more until realising that while hitting him doesn't achieve anything, showing him something asymmetrical does. He exploits this until Noah turns up and takes Kid away.
Ergo Proxy: An Entourage Auto Reiv belonging to Re-l Mayer, Iggy contracted the Cogito Virus and grew to become enraged at Re-l's lack of regard for him. He despises Proxies and blames Vincent for taking Re-l - his sole reason for existence - away from him. He develops an unusual state of both loving and hating Re-l, and locks her up to "protect" her, while he attempts to kill Vincent. However, an Auto Reiv belonging to a Proxy that Re-l killed, attempts to kill Re-l at the same time, causing Iggy to return to Re-l. The Auto Reiv goes into a self-detonation mode and destroyed much of Iggy except for part of his head (which left Iggy "conscious" to some extent). In the end, not wanting to see Iggy suffer due to the virus, Re-l shoots him and buries his head.
Threads Of Time: Sali Tayi comes to this conclusion very quickly when he discovers that Atan Hadas likes Moon-Bin.
Paprika: Osanai tries to strangle Konakawa in his dream when the latter tries to take Chiba, whom Osanai loves, away from him.
Kawaii Akuma: Naruse basically considers doing this to anyone who tries to get close to Akiyoshi. For example, he once threatened to stab a guy in the eye and later stabbed him in the chest when he found out he was hitting on Akiyoshi. And in another scene he comments to a group of students, "If any of you touch him, I'll kill you all. This guy belongs to me."
Shiki has Megumi who after turning into a vampire murders her crush's best friend, because she's jealous of how close they are.
Karakuridouji Ultimo: Rune, Yamato's lover in a past life reincarnated as a man, threatens to do this to Yamato's crush Sayama if Yamato doesn't do as he says.
Ibitsu: The Strange Lolita's solution to the fact that Kazuki already has a younger sister is to personally make sure that Hikari is taken out of Kazuki's life... permanently.
Tayutama has a minor case with quasi-tsundere Ameri, the Unlucky Childhood Friend of the main character. As Ameri watches Mashiro slowly win Yuuri's heart, she decides (with no small amount of influence from the Dragon) that Yuuri shouldn't have to risk his life with all this magical nonsense. She concludes that the only way to keep Yuuri safe is to drive Mashiro away... with a little help from the villain. In case this you didn't realize this a bad call, she gets bat wings, Dull Eyes of Unhappiness, and a dark purple aura.
This is essentially the main conflict in the second half of Da Capo, as Sakura tries to stop herself from subconsciously using her magical powers to either kill Nemu or drive her into insanity.
In the Trigun manga, this turns out to have been over half the point of the entire Gung-Ho Guns plotline. Legato was jealous of Vash. Because Knives was preoccupied with his twin, the only person on the planet he didn't intend to kill, and Legato apparently felt that if Knives had no one living to care about, he might find some scrap of affection for his chief minion. And Knives breaks his spine when he finds out.
Subverted in Air Gear. It seemed that Ringo Noyamano had finally snapped on Simca — enough to attack her with unusual violence to get her out of the way in the "race" for Ikki's love, but then we find out she was putting the Migratory Bird out of action so she wouldn't hurt or kill herself trying to participate in the war for the Sky Regalia, due to her lack of combat skills.
Tenchi Muyo! plays it for laughs. Ayeka and Ryoko are involved constantly in this with each other, while their power levels makes it closer to Wile E. Coyote vs. Wile E. Coyote. In Tenchi Universe it's played more seriously: Ryoko tries to convince Tenchi to abandon a kidnapped Ayeka to Kagato, possibly to invoke the trope, but when Tenchi chooses otherwise she decides to support him anyway.
Urusei Yatsura: Ran's default reaction to anyone who gets within ten feet of Rei.
In chapter 64 of Black Butler Undertaker attempts to kill Sebastian because he is making Ciel "miserable." He doesn't get a chance to. It's also worth noting that Undertaker has a considerable amount of Ho Yay towards Ciel and tends to lose his personal space around him.
In episode 17 of the first anime Sebastian has sex with a girl named Matilda to get information. When she finds out, Grell is shown reacting jealously and writing down the girl's name in a Death Note.
D.N.Angel: In episode 23 Kael to Elliot so that he can have Freedert for himself.
Mayuri Konishi/Sarashina from Bungaku Shoujo has traits of a yandere character. She is crazy about Akutagawa, with whom she went to elementary school. She admits being jealous of Kanomata, who Akutagawa used to protect. When she suspects Akutagawa has feelings for his classmate Nanase Kotobuki, she says: "If so, I'll cut Nanse Kotobuki into ground beef!!!".
Reimei No Arcana: Loki has quite a few elements of Yandere, from threatening Caesar for insulting Nakaba and regretting that he didn't kill Caesar due to Nakaba's and Caesar's growing closeness in earlier chapters to killing Bella for threatening to have Nakaba either killed or raped by soldiers.
Nova in Magic Knight Rayearth wants Hikaru all to herself and won't hesitate to kill her friends in order to eliminate all competition.
Depending on how you interpret their relationship, Griffith may have done this to Casca during the Eclipse to get to Guts.
Also, Dark Magical Girl Rosine. Platonic or not, she has a very strong obsession with having her Morality Pet Jill stay with her after being separated for so long. Which she demonstrates by telling Guts that Jill is "hers" after realizing that Jill has somewhat of a crush on him. While attempting to kill him.
Misa Amane from Death Note once told Light "If I see you with another girl, I'll kill her!"
There's also Kiyomi Takada, who was once Light's girlfriend. She really hates Misa for her involvement in Light's life and has suggested to Light that he should kill her.
And then when Misa and Takada have dinner together, Misa says that if Takada goes too far, she'll get the death penalty.
Aria the Scarlet Ammo: Shirayuki Hotogi. A perfect Japanese beauty according to Kinji, except for one flaw - jealousy, which turns her into a berserker who assaults any girl approaching Kinji. When she finds out that Aria is living with Kinji she goes as far as trying to murder her. She is shown to love Kinji to the point that she will do anything for him.
In Lovely ComplexMimi has a crush on Otani. She then learns that Otani is dating Koizumi and acts incredibly malicious towards her, including telling her with a Slasher Smile that she will make whatever is left of her life extremely painful if she gets any closer to Otani. She later mellows out though more or less.
Nanami does exactly this to a kitten in Revolutionary Girl Utena when her brother pays more attention to it than her, and she was the one who got him the kitten for his birthday in the first place. She also knows her brother is interested in Utena and actively tries to kill Utena during a duel with her.
In one episode, Miki's piano teacher is hinted to have a less than savoury interest in him, and he's inappropriately touchy-feely with him when they talk. Miki's twin sister Kozue has clearly noticed, as it later turns out that she'd previously shoved the teacher down the stairs for it.
Highschool of the Dead: Takashi believes he may have crossed the Moral Event Horizon after being forced to kill his best friend, Hisashi. Though it was to protect himself and Rei, Takashi admits that it was also partly out of jealousy because Rei had chosen Hisashi over him.
Manami Anzai from Life, who first appears as a cute and childish click leader who snaps utterly when she suspects Ayumu is trying to steal away her boyfriend. She goes so insane that she then dates a gang leader on the side to use him to get back at both Ayumu and her boyfriend, and then promptly screws her boyfriend and the gang leader both over. Not to mention her vow to kill Ayumu with her bare hands. Then she crosses intoCute and Psycho territory.
Jagara from Wolf's Rain killed her twin sister Hamona because she was in love with Hamona's husband Lord Darcia. Darcia did not appreciate Jagara's attempt to "free" him from the responsibility of caring for his comatose wife.
Possibly Ryuuko Konuma's motivation for selling out Aoshika to Haguro beside, you know, avoiding ANOTHERbeating. Heaven help Aoshika.
Given the eighteen chapter gang rape, heaven's a little late.
After THAT, Haguro also tries to kill Aoshika while telling her that Inugami is too good for her. Fortunately, Inugami shows up in time.
Honne-Onna of Hell Girl has diehard fangirls. One was ready to send someone to hell because she thought he was dating her. And another actually sent the other girl to hell because she thought she got too close to Onna.
Hiromasa of Love Pistols considers doing this in regards to anyone who expresses interest in Shima.
Warau Hyouteki aka The Laughing Target: When Azusa and Yuzura were mere children, they were betrothed to each other by their parents. Many years later, after Azusa's mother mysteriously dies, Azusa returns to Yuzura with the intent of marrying him; Yuzura has a girlfriend, Satomi, and this doesn't sit to well with Azusia. Add to the fact that Azusa is anything but "normal", and she has no problem eliminating the competition by any means possible.
In Fate/Zero, Kariya Matou plans to do this with Tokiomi (the husband of his unrequited love Aoi). It's not purely for this reason, since Kariya blames him for handing him and Aoi's daughter Sakura over to the Matou's tortures (Though neither Aoi nor Tokiomi actually know what she'll be put through — the Matou leader, Zouken, is that much of a Bitch in Sheep's Clothing). Unfortunately, Kotomine kills Tokiomi first, but he sets Kariya up so Aoi (who actually does love Tokiomi) is led to believe Kariya did do it, and things get... messy.
Played for Laughs in episode 5 of Sakura-sou no Pet na Kanojo, Sorata and Mashiro have a talk after Nanami agrees to move into Sakura Hall. Only Sane Man Sorata's plan it to bribe Mashiro with food with a promise that she would take care of herself. Jin the Casanova suggests they tell Nanami they're dating, which Sorata immediately rejects. Innocently InsensitiveIdiot Savant Mashiro then innocently invokes this trope by stating that she should kill Nanami to keep their secret safe, which he also immediately dismisses. Sorata decides to try his idea, which he calls Plan A, calls Jin's idea Plan B, and Mashiro suggests her idea is Plan C, causing Jin to start laughing wildly at Sorata.
During the "Return of the King" storyline in Ultimate X-Men, after Cyclops goes missing on a mission to the Savage Land with Wolverine, rumors start floating around that Wolverine killed Cyclops so he could be with Jean Grey. But it turns out that the rumors are, in fact, completely true. This comes as a special surprise if you're used to the main universe's Wolverine, who goes by samurai honor codes and such.
Strange example: Thanos cursed Deadpoolwith immortality. The catch? Thanos's love interest, Death, is herself in love with Deadpool and the feeling is mutual. By keeping Deadpool alive indefinitely, he won't be able to cockblock Thanos.
One EC Comics story had two stories in one involving the same characters: Murder the Lover and Murder the Husband. In the first, the husband finds out his best friend is having an affair with his wife, and invites the friend up to his forest cabin with murder on his mind. In the second, the husband is ignorant of the affair, and the friend takes advantage of an innocent invitation up to the forest cabin to get the wife for himself once and for all. As predictable in EC comics, neither plot ends well for the perpetrator.
A recent indie comic, The Two Sides of Jack, has a subversion. The plot is Jack's girlfriend has been cheating on him, but her new lover is now a zombie. After going through a massacre (Finding that it's not easy when you're overweight) he eventually manages to complete his mission. He then cuts off the guy's dick and sticks in his girl's mouth, telling her "Fuck off and die bitch." Quite a memorable ending, and squicky at that.
One time, The Joker messes with a rehabilitative Two-Face, suggesting that his girlfriend and best friend Bruce Wayne are playing behind his back and want to keep him locked up in Arkham. To give a further push, Joker has one of his minions make a fake newspaper article that Bruce and Two-Face's girlfriend are going to marry and slips it to an already suspicious Two-Face, who now loses it, escapes from Arkham, kidnaps his girlfriend and tries to kill Bruce, fulfilling this trope himself.
This is the premise of many fanfics, in that one character attempts to kill a second to get possession of their love interest. What usually follows from this is that the third character rides to the rescue of the second and fluffy romance and/or sex ensues.
Hikaru does this in the infamousKimagure Orange RoadRevenge FicRevenge Road, but gives the hypotenuse, Madoka, a relatively merciful death by being shot in the head, noting that even if she stole Kyosuke away from her, she was her friend for a long time. Kyosuke, however, is killed in a far more brutal manner, being tortured to death with knives.
In the Hans Von Hozel story Valentine's Day, Harry sets a dragon on Ron so he can have Hermione for himself.
In another MLP story called Split Love, Pinkie sees Applejack and Rainbow Dash kissing and decides to kill AJ out of jealousy.
In the Lost Girl fic Faethful the Garuda encourages Dyson to murder Lauren, and clear the way to Bo. He refuses, but Lauren is understandably rattled.
Unfortunately, this leads to consequences in the sequel. The combination of him having his love returned and the Garuda's challenge causes him to undergo a wolf Fae right-of-passage (albeit an extremely perverse version). He spends the sequel trying to show his "dominace" over Lauren to demonstrate he is Bo's true lover. He very nearly succeeds twice, before Bo beats it out of him.
The Book Of Life features a rare example of an outside party doing the murdering. When it looks like Xibalba is going to lose The Bet, he has Manolo killed via snake bite. While it technically works, it turns out to be far from the last word on the subject.
In Tangled, just because her love for Rapunzel is non-existent or at best a warped and possessive thing doesn't mean that Mother Gothel can't resent and stab Flynn mortally because he and Rapunzel love each other, and his actions resulted in Rapunzel realizing she doesn't love Mother Gothel.
Films — Live-Action
Orphan: All of Esther's actions in this film have in mind the ultimate death of Kate, Danny AND Maxine so that she can have John all to herself.
In Batman & Robin, Poison Ivy deactivates Mr. Freeze's Ill Girl wife Nora from her life support system and blames it on the heroes - and in the process, she's able to convince him to take revenge on all of humanity by freezing the world and leaving her and him as the only people alive. Freeze learns, once his plan is undone, that the good guys actually saved his wife and that Ivy was the one who tried to kill her, prompting a Heel-Face Turn. Clearly she is seriously into him.
Caledon Hockely in Titanic comes to this conclusion when he finds out that his fiancée Rose loves Jack more than she loves him.
In Howard Hawk's 1939 film, Only Angels Have Wings, Cary Grant's character orders a pilot to fly in foggy weather so he can have dinner with the other pilot's date. He ends up ordering the man to return to the airport, but after an unsuccessful landing attempt, he tells him to simply fly around until the fog lifts. The man is too eager, however, to make that dinner date, and ends up crashing his plane and dying. It isn't exactly murder—but when Cary Grant's character shows absolutely no regret for his actions and also says the other pilot wasn't a good enough flier otherwise he would have survived, it comes across as especially cold.
Lady Kaede in Ran, as part of her scheme to bring down the House of Ichimonji, manipulates Jiro to murder his wife, Lady Sue.
In The Crush, Adrian loves Nick so much she's willing to kill his girlfriend and her own friend to get him. Luckily, she doesn't succeed.
Debbie of Devil In The Flesh does this when she kills numerous people she thinks might try to prevent her from being in a possible relationship with her teacher, including her grandmother. She also kills her friend and tries to kill her teacher's wife as well.
In Devil In The Flesh 2 Debbie escapes from the mental institution she was sent off to in the first installment of the series. After the young co-ed dies in an unexpected accident, Debbie steals both the girl's identity and her car, and heads off to the college her victim was supposed to attend. There, Debbie quickly develops a psychotic crush on her dashing writing professor, Dr. Sam Decker, killing anyone she perceives as a threat to their relationship. However, Debbie's scheme starts to fall apart when her roommate begins unraveling Debbie's lies and discovers the truth about her past. Knowing that she is in danger of being exposed, Debbie makes plans to get rid of her roommate and her friends by any means necessary.
In Moulin Rouge!, The Duke threatens to do this in order to blackmail Satine.
The Duke: Tell her that the show will end my way and she will come to me when the curtain falls... or I'll have the boy killed.
In the 1925 film, The Phantom forces Christine to choose between two levers: one which will save Inspector Ledoux and her lover Raoul and one which will blow up the Opera. Christine of course chooses that lever that will save the two men, but it's a trick, of course. It will "save" the men from being blown up by drowning them. Christine begs the Phantom to spare them, promising him anything, and, in the end, The Phantom is unable to go through with it and opens a trap door that releases the water Just in Time.
Attempted with extremely little forethought by Noah in The Village when he learns that Ivy and Lucius are engaged. Naturally, not being possessed of full mental faculties, he sort of overlooks the "make it look like an accident and, with Lucius out of the picture, win Ivy over" part, but he does the "stab Lucius over and over again" part quite well.
In Scarface (1983), Tony Montana murders his best friend Manny Ribera, believing that he slept with his sister, Gina, for whom he harbors secret desires himself. However, she reveals that she and Manny are married.
There is an interesting take on this trope in the opening scene in Saw 3D. A woman has been involved with two men, neither of which know about the other. All three of them are put in a jigsaw trap where one of the three will die. The two men are forced to either kill the other man, or, together, kill the woman.
The message that plays before the "game" begins even states that "one of the sides will drop out of this love triangle". Ultimately this trope is averted since neither of the guys (hypotenuses) are killed, and instead the guys decide that the woman should die.
The House of Yes inverts this. Jackie, the Ax-Crazy lover of the main character kills him instead and lets the hypotenuse run away.
Deathwatch (1966): Petty thief Lefranc competes with his cellmate, Maurice, for the attentions of the murderer Greeneyes. Greeneyes, unaware of either man's infatuation, constantly switches his favor from one to the other, further escalating the hatred between the two. In the end, Lefranc strangles Maurice to death.
In The Haunted Mansion: Gracey pulls a sword on Jim and threateningly approaches him when Jim (Sara's husband) comes to rescue Sara from Gracey and Sara's wedding. But Jim pulls out a letter from Gracey's dead lover and Gracey backs off.
Madison turns it Up to Eleven with her crush on Ben in the 2002 movie Swimfan, killing two police officers and nearly killing Ben's girlfriend before she's finally stopped.
In 1996 Fear David is one hell of a Yandere towards his girlfriend Nicole. He quickly shows his true colors when he beats a friend of hers to death simply for walking with her and then goes totally ballistic after she tries to break up with him. He even tried to break into her house and kill her family.
In the French film Haute Tension, this trope is exemplified in the protagonist, Marie, who creates an alternate personality to brutally murder the family of her lesbian crush, Alex... simply so that she could have her all to herself.
Lost Highway. Mr Eddy: Her name is Alice. I swear I love that girl to death. If I ever found out someone was making out with her. I'd take this - pulls out a pistol - and shove it so far up his ass it would come out his mouth. Then you know what I'd do? I'd blow his fuckin' brains out.
The jack in the box in the Steadfast Soldier sequence of the Fantasia movie tries to kill the titular soldier because the ballerina loves the soldier and not him. Luckily, he doesn't succeed.
Play Misty for Me: Dave (Eastwood) is a womanizing DJ who constantly gets calls from a woman asking him to play the song "Misty". He eventually meets the woman, named Evelyn, and has a one-night-stand with her. Evelyn insists on making herself part of Dave's life and, at first, Dave likes the attention. However, his ex-girlfriend, Tobie, moves back to town and Dave decides to patch things up. After he attempts to tell Evelyn that he's not interested, she responds by attempting suicide. Dave and Tobie get closer, but Evelyn isn't going to take it and tries any way she can to keep Dave to herself, even if it means getting rid of Tobie for good...
Dark Shadows: Angelique hypnotises Josette and forces her to fall off the cliffs.
The Flying Brick title character of My Super Ex-Girlfriend (Uma Thurman) vows to destroy the protagonist's life after he breaks up with her. One memorable scene had her throw a shark into the bedroom of his new girlfriend after spending the night with her.
In Victor Hugo's The Hunchback of Notre Dame, Archdeacon Claude Frollo attempts to stab Captain Phoebus to death when he sees him kissing Esmeralda, the gypsy girl whom Frollo is madly in love with. Phoebus survives, and Esmeralda is arrested on accusations of witchcraft and murder. Frollo uses this as an oppertunity to confess his love to Esmeralda and tells her he killed him out of love for her. Esmeralda, understandably, is less than impressed and refuses to go along with him.
He paid for it. Oh, did he pay for it. When God actually calls you out and then the first child you have with your now-wife dies... Then your daughter gets raped by her brother (though according to Jewish law they weren't actually related...whatever). Then another son kills that son and rebels against you...and so on.
Agatha Christie used the trope in Sparkling Cyanide. The initial murder is committed by the victim's husband's secretary, who is in love with the husband and believes he would turn to her if the wife was out of the way. He doesn't, so in order for her murder to not be in vain, she engineers a convoluted series of follow-up murders designed to eventually give her control of the family fortune.
The Murder the Hypotenuse plot in Sparkling Cyanide was almost successful; it's strongly implied that the husband and the secretary would have gotten together eventually (with even the victim's relatives commenting on what a good couple they would make) except he accidentally drinks poison that was intended for his sister-in-law.
The Sherlock Holmes story The Crooked Man features a military officer who betrays an underling to the enemy so he can steal his girl. Unlike the David and Bathsheba legend (which the story references), the underling survives (despite Cold-Blooded Torture from the enemy). His condition is such that he avoids his old love out of fear of pitying him, but he comes back eventually, his love recognizes him, and the false husband dies of stroke on learning of it.
The major subplot of the novel A Civil Campaign featured Miles Vorkosigan being accused of doing exactly this during the events of the previous book Komarr (in order to free up the woman he was now attempting to court) by political opponents. The truth being part of an ultra-classified incident that couldn't even be revealed to some of the highest levels of government (if the bereaved hadn't been a direct witness, she wouldn't even have known), so things got rather... complicated.
Instead of getting killed, Edmond Dantes, a.k.a. The Count of Monte Cristo gets sent to the hellish prison named Chateau d'If, all thanks to a conspiracy mostly engineered by his "best friend" Fernand so he can have Mercedes, Dantes' fiancee.
That's the film adaptation with Jim Caviziel. In the original book Fernand is barely even an acquaintance of Edmond. Fernand is in on the plot and does not hesitate to take advantage.
In the Ben Bova novel Mercury, the story starts with Mance Bracknell, lead engineer on a space elevator, married to Lara Tierney. Victor Molina, astro-biologist best friend of Mance, wants Lara. When the space elevator collapses, killing hundreds of millions of people and devastating many nations, Victor falsely testifies in court that Mance was negligently responsible for the collapse, getting him put into forced labour for life. After an accident wipes out everyone on a spaceship but him, Mance takes on the identity of one of the crewmembers and (eventually) gets a job as manager of a colony on Mercury. He lures Victor (who is now happily married to Lara) to Mercury with some life-bearing rocks he's planted, with the goal of getting Victor publicly humiliated when his astro-biologist peers inevitably discover that the rocks aren't from Mercury (thus showing him as either a fraud or an idiot, either way ending his career). Mance seems to think this will get Lara back, but when he reveals his identity, plan, and Victor's false testimony, Lara rejects them both. Smart woman.
In Dan Brown's Digital Fortress, Susan Fletcher's boss Trevor Strathmore is in love with her and attempts to have her boyfriend killed to win her over. This backfires massively when she comes across his pager showing the hitman's report.
In Edgar Rice Burroughs's Princess of Mars, after Dejah Thoris believed John Carter died and promised to marry a foreign prince, she explains that they have laws to prevent this trope: she can not marry the man who killed her fiance. He carefully arranges that someone else kill him in their attack.
In Thuvia, Maid of Mars, Carthoris sees that a beleaguered ship is that of Kulan Tith, betrothed to the woman he loves, and is tempted to sail on, leaving him to his death. He doesn't, of course, even though Kulan Tith himself comments on the Heroic Sacrifice entailed.
In The Master Mind of Mars, when Ulysses Paxton revives Dar Tarus from Faux Death and proposes his man against Xaxa, Dar Tarus is eager: he reveals that, in the Back Story, Xaxa had sent him to this Faux Death so her favorite could woo the girl who loved him.
Geoffrey Clifton in The English Patient, when he finds out about the title character's affair with his wife, tries to kill him. And the wife. And himself. So it's really more "murder the hypoteneuse and the legs".
Victoria in Charlotte Dacre's Gothic novel Zofloya; or, The Moor poisons her husband to free herself to pursue his brother Henriquez, and then murders Henriquez's beloved in order to get rid of the competition.
In the Dragonlance series, Kitiara Uth Matar decides her romantic rival, the elven princess Laurana, is too beautiful to let live and plots to kill her. Kitiara ambushes Laurana in the city of Tarsis and with the help of the sivak draconian, Slith, manages to subdue the elfmaid but is then driven off before she can actually kill Laurana. Later on Kitiara succeeds in kidnapping Laurana and is about to have her subjected to a Fate Worse Than Death when their mutual love interest, Tanis Half-Elven, rescues Laurana. Tanis then chooses Laurana over Kitiara mainly out of disgust over Kit's attempts at killing her rival.
Done twice. The first time, it's adult vampyre Loren Blake in Chosen, and at the end of Tempted, Zoey's human boyfriend Heath gets the axe as well.
In-story, it's Dallas's motivation to try and kill Rephaim. "I'm gonna fry him. And then whatever weird control he has over you is gonna be gone. You and me can be together, and I won't tell shit about what happened here, as long as you're my girl."
The Sorrows of Young Werther plays with this trope a bit - Werther considers playing it straight and killing Albert, realizes he can't, and then inverts it by killing himself instead.
Blood And Chocolate: Rafe hopes to kill Aiden, though whether it's because he wants Vivian or because his relationship with Astrid has driven him to it is left unclear.
Though he doesn't go through with it, Edward Cullen of Twilight seriously considers doing in Mike Newton for daring to touch Bella, and probably thought about murdering Jacob Black for the same reason.
Jin Yong: Zhou Zhiruo made a huge effort to secure her position in Wuji's heart - including attempting to murder any and every woman that got in the way. And then there's Ah Zhi with Xiao Feng... let's face it, anyone who looks to be a nuisance or gets in the way of her and her brother-in-law must die.
The Tin Drum: Inverted. Agnes kills herself rather than choose between Matzerath and Jan. This way, neither of them has to lose her to the other.
In Gaston Leroux's The Phantom of the Opera, the narrator speculates that the mysterious figure Raoul shot on his balcony really was Erik coming to kill him, but circumstances forced him to change tactics. After Raoul comes to rescue Christine, Erik tortures Raoul to the brink of suicide in a Death Trap before turning it into a Drowning Pit, telling Christine that "she didn't need two fiancés," and only releases Raoul when Christine swears to marry him.
Tim the Ostler from Alfred Noyes' poem "The Highwayman", who is insane and deeply in love with Bess the landlord's daughter (and the Highwayman's love interest), is presumably the one who tipped off the Red Coats about the highwayman.
Eleyna in the Star Trek novel "The Vulcan Academy Murders". Sendet as well was at least considering it.
It's implied, but not outright stated, that Littlefinger had this in mind for Eddard in A Song of Ice and Fire, so he could steal his wife Catelyn. Mind you, he was probably aiming for 'disgrace and permanent exile' and not actual death: When Joffrey actually decides he wanted Eddard dead, that probably ruined Littlefinger's plan, but the man's actual intentions are still unclear so for all we know he wanted it to happen.
Mistborn contains an unintentional example- or arguably two. Vin kills Elend's ex-fiancee, Shan Elariel, who was attempting to kill Elend at the time, and later Zane in a triangle where she's Alice. Actually, that particular romance advances via death four times.
The Princess Bride: When Buttercup gives Prince Humperdink "The Reason You Suck" Speech, he throws her into her room. Then he races down to where her true love Wesley is being tortured, and sets the torture device on maximum, almost killing him.
Tinkerbell. Yes, thatTinkerbell. She did not by any means like Wendy, she even tried to kill her. All because Peter paid more attention to her than Tink. Most adaptations keep the general clinginess but tone it down from "homicidal fury." This also occurs in the film adaptations.
The mermaids were precious little better, but, then again, they don't seem to like anyone in the book (The Peter Pan Disney mermaids... well, they aren't much better than Tink. They do take particular distaste in Wendy whatever the version and try and drown her.)
In Tolkien's The Silmarillion, after betraying the city of Gondolin in return for its princess Idril, Maeglin tries to clean up the loose ends by killing her husband Tuor and son Eärendil.
Thérèse Raquin centers on Thérèse and Laurent killing her husband, Camille, so they can be together.
Linda Fairstein's first Alexandra Cooper novel, Final Jeopardy, features an erotomaniac woman who stalks and kills a movie star she perceives as standing between her and the object of her infatuation.
In Poul Anderson's Time Patrol story "Brave to be a King", Manse feels the temptation, when he doesn't even have to kill the man. According to Time Patrol rules, leaving Denison in place, which would preserve history, is the right thing to do. He'd live out his life, and his widow would doubtlessly grieve and recover.
The Cat Who series contains a book where Melinda Goodwinter tries to kill Qwill's love interest so that she can have him for herself, though it's unclear if she's really after Qwill or just his money.
The villainous Shoteka from Seeker Bears tries to kill Toklo so he could try and mate with his (Toklo's) mother. This is true in real life since male bears will sometimes kill a female's cubs to mate with said female.
In Those That Wake's sequel, What We Become, it's not murder, but close. Rose tries to replace Mal's memories of Laura with herself.
This backfires spectacularly on Lorenzo Mura in Elizabeth George's Inspector Lynley novel Just One Evil Act. He's with Angelina Upman currently, but when her daughter by a previous lover named Azhar goes missing Azhar comes to Italy to help in the search, and he's very put off by how the two clearly still have feelings for each other. So Mura hatches a scheme to poison Azhar using some E. Coli he procured from a friend in his wine — and problem #1 is that Azhar is a devout Muslim who doesn't drink and passes on it when offered. Problem #2 is that Angelina drinks some instead, despite Mura's outburst, in front of a news crew no less, that she shouldn't, but no one there thinks twice about it since she's pregnant and they figure that's why he's acting like that. Then Angelina gets sick and ultimately dies from the E. Coli, so Mura not only failed to kill his love rival, but also killed his lover and unborn child in the process. Sucks to be him.
In the Age of Fire series, it's speculated in-universe that RuGaard's Love Interest Nilrasha killed his arranged mate Halaflora so that she could have him for herself. However, she claims that Halaflora choked to death while eating more than she could chew, which given Halaflora's health, is entirely likely, and since the only other witness was Rhea, no one is ever able to confirm nor deny Nilrasha's story.
Inverted in the episode of Star Trek: The Original Series called "Requiem for Methusleah". In the episode, an immortal human named Flint creates an android mate called Rayna for himself. He has Captain Kirk date Rayna so her ability to love awakens. He succeeds too well. When Flint and Kirk fight over her love, she cannot handle the emotional conflict and it destroys her.
In the Star Trek: Voyager episode "Fair Haven", Captain Janeway finds herself attracted to the program's resident bartender Michael Sullivan...until she finds out that his character is married. Janeway decides to do a little "reprogramming" so that the wife never existed in Sullivan's life.
An episode of Law & Order features the rather creepy example of a man who, obsessively in love with his best friend's wife, murdered him and framed it to look like an accident; the grief-stricken wife later independently fell in love with and married the best friend, completely unaware that she was marrying not only the man who murdered her husband but her own stalker.
Another episode deals with a woman with erotomania who kills the wife of the man she's obsessed with. That she's otherwise incredibly intelligent, cogent, and calm leads the police to believe she's actually having an affair with the man and that they conspired to kill his wife.
In LOST, resident Magnificent Bastard Ben Linus had a crush-turned-obsession on Juliet. She, on the other hand, was with Goodwin. You don't need a degree to figure out what happens to the poor fool.
In Rome, it's pretty much standard procedure to kill someone in the event of any romantic complication. Need your daughter for a political marriage? Have her husband killed. Found out your best friend's wife has a child by another man? Kill him and dump him in the sewer. Want a married man? Poison his pregnant wife!
Found out the slave that you just freed in order to marry her is already engaged? Bash her fiancé's brains in!
Alpha in Dollhouse takes this Up to Eleven - he tries to kill every person who loves Echo. However, he focuses particularly on Paul Ballard, because he's the only one she feels the same way about.
Alpha does get some of his marbles back in the series finale, taking place in the Bad Future. After Paul is killed, he leaves Echo a mental copy of the guy, so they can be together, sort of.
In Desperate Housewives, this happens when Bree tries to get back to her cheating husband Rex by going out with George, the local pharmacist and Stalker with a Crush. Eventually she realizes that she considers George just a friend and goes back to Rex, who had recently had a heart attack. George then continuously switches Rex's pills until he dies and then proceeds to date Bree again.
Ryan O'Reily orders his brother to kill Dr. Nathan's husband, thinking he could win her over if her husband was gone. Sad thing is that he was right. The fact that he also killed the man who raped her, which is possibly the only time he's gotten his own hands dirty as he usually manipulates others into doing the deed for him, had a lot to do with it.
Chris Keller, when he basically kills all of Beecher's previous lovers as well as his father's murderer.
Noah's Arc: It's strongly implied Guy is planning to kill Alex and make it look like a suicide in order to be with Trey (note that Trey is totally oblivious to any of this.
A deliberate example of this is attempted by a character in The Sarah Connor Chronicles.Jessie pushes Riley toward John in an effort to force Cameron to consider her a threat, and therefore force Cameron to kill Riley, all in an effort to alienate John and Cameron.
Lana tried to kill Chloe once. No, really. Mind Control.
The Jimmy-Chloe-Davis triangle of Season 8 ended in this way, with an attempted use of the trope by Jimmy, a successful use of the trope by Davis and a last-minute equalization of the sides, leaving Chloe all alone. If Chloe goes dark, this will be where it started.
Many girls also try to kill Lana, Chloe, or Lois to have Clark for themselves.
Maxima: (about to punch Lois) I finally found the man I waited for all my life, and you can't have him!!!
Battlestar Galactica: Galen seems to have his own little cycle going with his girlfriends. Cally kills Boomer. Tory kills Cally.
And finally, Galen kills Tory, although admittedly, Tory never seemed to be very into Galen, and more into a rather warped sense of self-preservation. Either way, hope it was worth it girls.
Freddie's fate at the end of S4 of Skins(killed by Effy's deranged psychiatrist).
In Robin Hood, Kate picks up a bow and arrow and deliberately tries to shoot Isabella dead, her rival for Robin's affections. Interestingly, when Isabella tries to drive a wedge between Robin and Kate, she resorts to non-violent trickery and uses a broken locket to make Kate believe that Robin is cheating on her. Now guess which one's the hero and which one's the villain.
In the 1998 series Merlin, Merlin attempts to defy this trope by convincing Uther not to kill the husband of Igraine, but Uther does it anyway.
In the other TV show of Merlin, Arthur snaps and tries to kill Lancelot after he catches him passionately kissing Guinevere on the eve of their wedding. Only Gwen and Merlin's intervention saves Lancelot's life.
Penelope attempts to do this on Sonny With A Chance. She replaces a cheeseball that Sonny needs to fire from a cannon at her high school's homecoming with a cheesebomb, planning to have Sonny be killed so that Chad would be free to date Penelope instead. Chad refuses and survives Penelope's If I Can't Have You plot, getting to Wisconsin along with the So Random cast in time to save Sonny.
In one episode of Bones, a female suspect who had previously stalked the victim becomes amoured with Booth after he had briefly comforted her. Once she saw that Booth paid more attention to Bones (naturally, of course), she brought a gun with the intent on killing Bones right there and then.
One episode of Flashpoint deals with a husband and wife who had tried unsuccessfully to have a baby multiple times. The wife mistakens her husband's frustrations and efforts to convince her to give up for cheating on her with his ex-girlfriend She wasn't entirely wrong and promptly tried to kill the girlfriend.
A twisted one in the episode "Seven Seconds" of Criminal Minds occurs, where the kidnapped girl was found to be molested by her uncle and was kidnapped and almost killed by her aunt to "protect" the family, though Prentiss implies that it was also to make up for the fact her husband didn't want her as much compared to her niece.
In Kamen Rider Kiva, Wataru and Mio are in love, but Mio is due to have a forced marriage to his brother Taiga. Wataru refuses to kill Taiga, so Mio attempts to do it herself by running him through with a sword at their wedding. This proves fatal not for him, but her - he survives and she is killed by Bishop.
In The Walking Dead, Shane tries this in season 2 by luring Rick out to secluded field where he can shoot him and pin the blame on another person he killed. He tries to justify himself by claiming that Rick is a weak man and a weak father who isn't willing to get his hands dirty fighting for his wife and children. Rick proves Shane very wrong moments later by pretending to hand Shane his gun only to gut him like a fish.
Regina Mills (Queen Regina, in Storybrooke) uses the last of her magic to grab a poisoned apple from the Fairy Realm to bake a special apple turnover for protagonist Emma so she can get rid of her adopted son's biological mother and have Henry all to herself. Henry immediately intervenes and eats the turnover to protect Emma, falling into a magical coma, forcing his biological and adopted mothers to work together to break the spell.
Glee, being primarily a comedy show, plays with this and has several characters trying to do this in various dramatically high school-minded and therefore non-lethal ways. Of note is Sebastian, who tried to blind Kurt with a slushie that was laced with rock salt. Blaine - the Alice in the situation - ends up taking the slushie instead. The incident, in fan-theory, leads Blaine to avoid and become guarded around Sebastian and treat him more like an actual threat like everyone else does. Oops.
The Vampire Diaries has an interesting subversion. Tyler and Caroline are blissfully in love, but Klaus wants Caroline. Tyler leads a rebellion of hybrids against him, and Klaus' extremely unforgiving nature demands that he kill Tyler. Klaus ultimately refrains, realizing that it would fall under this trope and he'd never have a chance with Caroline.
In Copper, Eva Heissen slit Molly's throat when she found out about her pleasuring Kevin.
This happens sort of incidentally in volume 16 of Ciel The Last Autumn Story. Lariatte was Krohiten's main competition for being Yvienne's most important person, but the actual reason he helped orchestrate Lari's death was to use Yvienne's response to her death as part of a plan to prevent the predicted extinction of mankind. And the death wrecks their relationship anyway.
Essentially the theme of the song "Rocky Raccoon", by The Beatles. Between that song and "Run For Your Life," you really have to wonder about the Beatles' relationships.
"Jenny Again", by Tunng. The titular Jenny is the equivalent of the example's Alice: the listener is put into the position of Bob, and the singer is in the position of Chris.
A woman threatens False Rape Accusation to coerce her lover into this in "Pandora" (though it's implied to be more about her husband's life insurance than love.)
The Story of Evil song series in the Evillious Chronicles has an event where Princess Riliane is so furious that King Kyle rejected her proposal of marriage because he was in love with a green haired woman in Elphegort that she decided to murder all of the green haired women in that country because she didn't know which one he was in love with. It's brought up in the tie-in novel as the main reason her kingdom's citizens decided to rebel.
In Flash Gordon, in the city of the hawkmen, the king's favorite goes to murder Dale when she is sent to the harem.
A bizarre example happened not once, but twice in Peanuts; Lucy started to think that Schroder's piano was a sort of "competition" that kept him from paying attention to her, so she tried to destroy what she saw as her "rival" on two separate occassions, the first time throwing it to the Kite Eating Tree and the second time throwing it down the sewer. While technically she destroyed the piano both times, it didn't help; Schroder just got a new one each time. (And the second time, threatened to slug her if she "scratched it with her stupid elbows".) It would seem that the piano was not the problem at all (as anyone who knows Lucy can surmise.)
Happens in the continuing example game in Bliss Stage, prompted by Josh asking Keenan what he thought of the love interest Sara. His response: "Man, she's getting all girly and clingy and shit. Lousy lay, too."
In the far-flung history of the the Clans in BattleTech, Khan Ellie Kinnison of Clan Steel Viper was obsessively in love with the Clans' supreme leader, Ilkhan Nicholas Kerensky. However, Kerensky was married to Jennifer Winson of Clan Wolf. Kinnison had the brilliant idea of trying to murder Winson so that a grieving Kerensky would turn to her for solace. Only timely intervention of Kinnison's saKhan Steven Breen spoiled the plot. Trying to assassinate Winson while she was traveling with Kerensky probably didn't help her case either. Kinnison was killed in the attempt on Winson's life by Breen who shot her and received a Klingon Promotion.
Medea: The original Woman Scorned, she killed her husband Jason's new fiancée after being dumped for her. This wasn't an attempt to win him back, however, but pure and simple revenge, particularly when she also put her children by Jason to the sword just to make him suffer all the more.
In Sweeney Todd: The Demon Barber of Fleet Street, Judge Turpin fancies Benjamin Barker's wife, and has him convicted of a crime he didn't commit and shipped off to Australia to get him out of the way. However, as in The Crooked Man example above, he escapes from prison and returns to get his revenge on the judge.
Arguably, Mrs. Lovett, with Sweeney and Lucy. She doesn't really do anything to Lucy, but still she lies and claims she's dead, just so she can have Sweeney all to herself.
Attempted in The Crucible. Abigail Williams is in love with John Proctor, and tries to get Proctor's wife Elizabeth hanged as a witch. Due to a bit of Deus ex Machina, she not only fails (Elizabeth is pregnant and can't be executed), but she gets Proctor himself hanged instead as he chooses to take the heat. Oopsie.
In Little Shop of Horrors, Seymour almost does this with Audrey's boyfriend Orin. He loses his nerve, but Orin dies immediately afterward through his abuse of his laughing gas and Seymor's inaction. Of course, this is less because he's dating Audrey and more because he's abusing her.
Arguably, Audrey II him/her/itself attempts this when it tries to off the original Audrey, depending on how you interpret the plant's relationship with Seymour.
In a subversion, in Avenue Q, after Princeton breaks up with Kate and later gets together with The VampLucy the Slut, Kate angrily throws the lucky penny Princeton gave her off the Empire State Building, hitting Lucy, breaking her neck, and putting her into a coma. Oops.
Princeton:Can you believe it? Some idiot dropped a penny off the Empire State Building.
Double subverted in The Women of Trachis. Lichas is reluctant to tell Deianira that her husband Herakles is madly in love with Iole for fear she'll become hysteric and do something drastic. Deianira actually just pities the girl, and understands that Love Hurts even for Herakles. However, not comfortable with the thought that having Iole in the house will compromise her family, she decides to use a love potion on Herakles to get his affections back, which is actually a poison that kills him.
A variation in Super Robot Wars W: Aria wants to kill Kazuma over his sisters. Makes a bit more sense when you learn that Aria is sort of an Opposite-Sex Clone of Kazuma, loving his sisters as if they were her own and hating him for having "her" place in the family.
One of the possible endings of a sidequest in Jade Empire, best described as "childhood promise gone wrong". Once you gather the poor man, the mobster woman he "promised" to marry (at all of about eight), and his fiancee, you could convince the mob boss to call off her pursuit... or, convince her to kill the fiancee. Which she does. At which point, the man rightfully objects - so she kills him. Finally, having realized she didn't mean it, she tries to kill you. Good job, jerk.
In Baldur's Gate II, NPC Aerie can be romanced by both the player character and fellow NPC Haer'Dalis. If she chooses you, he'll back down, but if her romances with both proceed at about the same pace, he tries to settle things in this manner.
It can also happen if you keep Viconia, Jaheira, and Aerie in the party at the same time and don't break off the romance with any of them.
The bad ending of Irisu Syndrome stands out in this regard, since one point of the triangle kills everyone in the vicinity, including the person she originally loved.
Riku tries to do this to Sora in Kingdom Hearts. Sora wins and Riku lives, but Riku winds up possessed by Xehanort's heartless. The two do reconcile at the end of the game, once Riku is back in his right mind.
In Chrono Cross, this is apparently played straight when Karsh murdered Dario in the Isle of the Damned. Subverted when it turns out that Dario was possessed by Masamune at that time, and when he declared his intent to slate the Masamune with Riddel's blood, Karsh went berserk and struck him down.
Happens in Rule of Rose as a testament to the Love Makes You Evil-trope: Jennifer and Wendy swear eternal love and devotion to each other, but then Jennifer finds a puppy and decides to keep it. Wendy doesn't want to share her affections with a "filthy animal", so poor Brown is toast, along with their relationship. For bonus points, they were maybe ten years old at this point.
Toward the end of Kain's miniature dungeon in Final Fantasy IV Advance, he is given the choice of killing Cecil or facing Dark Bahamut. Much of the previous trial highlighted presumed romantic jealousy that Cecil has Rosa's love, but the correct answer, choosing not to kill Cecil, signifies him coming to terms with this.
A double disturbing example in Assassin's Creed: Brotherhood comes from both Cesare and Lucrezia Borgia, Brother and Sister in an incestuous relationship. Lucrezia brutally beats Caterina, who is their prisoner, because she believes Caterina attempted to seduce her brother during the long carriage ride to their home (which she didn't). Lucrezia however, is in a relationship with a actor simply to make Cesare jealous, he orders his men to kill said actor.
Sodia tries to do this to Yuri in Tales of Vesperia because she's jealous of his bond with Flynn and thinks Flynn would be better off without his closest friend. Yuri, to his credit never actually tells Flynn what she tried to do. Probably because he knows what Flynn would do if he found out.
A variation of this exists in Team Fortress 2: Never touch the Heavy's weaponry or else he'll mow you down with it
Catherine: Katherine does this to Catherine (bear with me) with a knife in an animated cutscene before Stage 8.
Invoked and ultimately averted in BlazBlue, when Hazama uses Tsubaki's Clingy Jealousgirl tendences over Jin and amps them Up to Eleven. She then sets out to kill her best friend Noel upon the belief that she's stolen her rightful place next to him. But even then, once Tsubaki angrily curbstomps Noel and the physically/emotionally wounded Noel lies on the ground, waiting for the death blow... Tsubaki cannot bring herself to finish Noel off, and breaks down crying.
Cia starts a war with Hyrule in an attempt to kill Zelda and make Link hers. It goes as well as you think it does.
In Tsukihime, Arcueid tries it on Ciel in Ciel's route, SHIKI tries it on Shiki in Akiha's route, and finally Akiha tries it with Kohaku andShiki in Kohaku's route. Interestingly, in the Good Ending for Kohaku's route, Shiki realizes that the reason Akiha fails to kill Kohaku is that she wasn't really trying.
In Umineko no Naku Koro ni, Kyrie makes it very clear that, if Asumu hadn't died from other causes, she would have most certainly killed her within the year to be with Rudolf.
Kyrie: She was fated to die no matter what. The miracle was that I didn't have to get my hands dirty.
In one ending of Bionic Heart, Tanya kills Luke's girlfriend Helen because she is jealous of all the time Luke spends with her.
A mentally-broken Kotonoha snaps on Sekai and cuts off her throat on a bridge in broad daylight.
Sekai is the one who gets broken, then pushes Kotonoha in front of a train, and Makoto ends up with Kotonoha's torn-off arm in his hand when he fails to save her. (The original version of this ending came from the PS2 port, and a new version was made for the HQ remake.
Sekai tries to push Kotonoha in front of a train, but she ends up falling in front of the tracks and is hit, while Makoto manages to pull Kotonoha back up and saves her. (Kotonoha is seen pulling on Sekai's hair at some point, which "helps" with her death - it's unclear if this was intentional from Kotonoha or she did it while in a blind panic.
Sekai tries to push Kotonoha in front of a train, falls on the tracks, and both girls get hit by the train.
Szark considered killing Luna so he could be with Dominic in Dominic Deegan: Oracle for Hire. This was a residual effect of the demonic wound that had gotten him addicted to killing, and he resisted the temptation to even daydream. On the other side, Karnak really did try to kill Dominic's father in order to claim his mother.
Oasis from Sluggy Freelance is almost perpetually two punch daggers away from murdering even a vaguely perceived hypotenuse, and Torg lives in fear that it will eventually be Zoë, which it almost has been. The story "Fire and Rain" revolved around Oasis' deranged and Determinator stalking of Zoë. And then came "bROKEN"... Best not to even talk about that.
Torg enlists Bun Bun and the Angsty Dentist to teach him the stalking skills he needs to kill Zoe's boyfriend Leo. In the end, he doesn't go through with it because he just want Zoe to be happy. Bun Bun, however, knew he was too much of a nerdboy to go through with it the whole time.
Halfway averted in the love hexagon that is Starscream's Brigade in the Insecticomics. While Dreadmoon doesn't actively try to kill Skyfire, the partner of his long-time crush Starscream, he has occasionally directed some "friendly fire" his way and sent him threatening Mini-Cons.
Averted in Girl Genius: the Love Triangle is complicated firstly by the fact that A) they're all mad scientists, and B) they all have conflicting loyalties, but the two romantic male leads are Vitriolic Best Buds. Of course, a little romantic sabotage now and then is different.
Questionable Content never goes quite so far as to reach murder, barring the Vespavenger, but before Angus or strip 500, Faye was strangely violently possessive protective of Marten. Dora even mentions it the first time Steve and Ellen broke up that "[Ellen] has no idea how close to death she just came" when Ellen began a comment that started like she was about to ask him out.
In Kevin & Kell, Vin Vulpen makes two attempts on Rudy's life, jealousy over his relationship with Fiona being one of a few motives. He tries to mark Rudy with prey pheromones to ensure that his teammates accidentally eat him, but Rudy manages to survive. He later challenges Rudy to a duel, wanting to win Fiona back, but Rudy exposes his domestication.
The Nostalgia Chick tried to stab Obscurus Lupa in the chest for rejecting Todd again, even though Todd has no interest in the former and has always stalked the latter.
One of The Simpsons' "Treehouse of Horror" episodes centers around an house with AI that's in love with Marge. Homer explicitly points out that Marge would be available to "man or machine" if he were to die.
AI: Machine, eh? Homer: Yep, a machine!
Danny Phantom, where Vlad constantly tries to kill his "best friend" Jack in order to win his wife Maddie's heart. He also has a creepy, stalkerish relationship with Jack and Maddie's kid and main character Danny.
Subverted in Justice League Unlimited: Shayera Hol (Hawkgirl) and Mari (Vixen) are caught in a Love Triangle with Green Lantern John Stewart. He is currently with Mari, but he was with Shayera and everybody is aware that there are still unresolved feelings between the two of them. When Hawkgirl and Vixen are on a mission together (with Vigilante) they are caught in a trap and hunted down by Thanagarians who want to execute Hawkgirl for war crimes. When Vixen is captured she immediately strikes a deal with the Thanagarians — spare her and she will deliver Hawkgirl to them. She even goes so far as to cite the Love Triangle as the reason she will betray her superior so quickly. However, she does not go through with it, nor did she ever plan to; once the Thanagarians let their guard down and bring her to their aircraft she hijacks the ship so she can get herself and Hawkgirl out of there safely. Later episodes play the trope for laughs as Shayera and Mari begin to become close friends, as Shayera comments that she is trying to learn proper Earth protocol for these situations and thusly can not just poison Mari's water like she would on Thanagar.
The relationship eventually evolves into a quadrilateral by introducing Carter "Hawkman" Hall as a rival for Shayera's affection and the story wastes no time in getting to hypotenuse murdering: in Hawkman's second appearance, a physical manifestation of his dark side kidnaps John Stewart and orders Hawkman to kill him. Hawkman, being a hero, does not comply. In fact, the events of the episode cause him to stop pursuing Shayera romantically as he realizes that she and John Stewart are literally destined to end up together, despite what John Stewart says.
In the episode "Metamorphosis", a squicky example occurs when an extremely Over Protective Dad (who loved his daughter maybe a bit too much) didn't like his daughter's fiancee so he tried to kill him. When that failed, he pretended that the one who told him to do it was the fiancee's old friend Green Lantern, while showing him a photo of Green Lantern and his fiancee embracing. The fiancee then invoked this trope himself as he went after Green Lantern in a rage.
Played (relatively) straight in Family Guy when Stewie falls for his new babysitter, only to learn that she has a boyfriend. He then kidnaps her boyfriend, breaks his legs, ties him up and locks him in the back of Brian's car, ultimately leading to his death. Stewie then prepares to make his move on Ledan, only to realise that she's completely broken up about her boyfriend. Arguably subverted at the end where Stewie receives a gift from her and assumes that she does like him (of course, considering he's a baby and she's a teenager, this is hopefully not the case).
In Futurama, Bender says he assumed Fry was only pretending to love his old dog to mess with Bender's emotions (not a romantic love triangle, but love nonetheless). So, he throws the dog into lava.
Bender: Now I'm all you've got!
Of course, the dog is a fossil and thus already dead. Also, once he realizes that Fry genuinely loves his dog he dives into the lava to save the dog. You might even call it his Moral Event Horizon moment... if he weren't so comically evil.
Tragically, when Fry learns his dog had lived several years after its supposed death, he himself breaks the hypotenuse by refusing to resurrect his pet, insisting, "He forgot me a long time ago." In truth the dog had spent the rest of its life waiting in vain for Fry to come home.
Though, after the events of Bender's Big Score, this was averted.
In Benders Big Score Zapp Brannigan is all too keen on sending Leela's boyfriend Lars on a suicide mission to get him out of the way. Then again, Zapp is ALWAYS sending every single one of his crew members on suicide missions, so this is business as usual even if you discount his interest in banging a freshly grieving Leela.
Wendy, Stan's love interest/girlfriend in the early seasons of South Park, went so far as to have a substitute teacher arrested on false charges and locked inside a rocket launched directly at the sun, for the crime of having been the object of a brief crush from Stan. Even worse, the teacher was actually a lesbian, so she never had any interest in Stan, yet Wendy killed her anyway. What a bitch!
What makes it either better or worse (depending on how you feel of said twist) was that the episode originally intended to have Ms. Ellen actually be a spy and be sent to the sun for her crimes. Parker and Stone tacked on the ending with Wendy paying off the Iraqis, as they felt the original ending was too much of a downer and that it'd be funnier if Wendy conspired to kill her.
On Jimmy Two-Shoes, Heloise builds a robot double to be her assistant. She's very effective, but Jimmy falls in love with it. As you might expect, the robot didn't last the episode.
Sadira, from the Aladdin: The Series, subverts this; she wanted Jasmine out of the way, before she eventually got over Aladdin, but did not want her dead or seriously hurt, even explaining it in detail to a grunt.
Parodied in Phineas and Ferb in the episode "Suddenly Suzy" where we find out Candace is very aware of this trope and hides anything she thinks could be used to kill her before resident Yandere Suzy arrives.
What makes this really Squicky is that Suzy is Jeremy's sister. Younger sister. Way. Younger. Sister.
Generator Rex features that Creepy Child minion of Breach's. She's never named, but she's absolutely obsessed with making Breach happy and tries to kill Rex when she realizes Breach likes Rex more.
One episode of Kid vs. Kat featured Coop going on a hike with a group of his friends, including his Implied Love Interest Fiona and his Stalker with a Crush Phoebe. Being a rather terrifying Yandere, Phoebe spends the entire hike trying to get rid of Fiona, even locking her in an abandoned cabin and then leading the others further along the trail so that they wouldn't hear her calling for help.
Fosters Home For Imaginary Friends: One episode featured a newcomer called Berry. Berry is initially sweet and polite, but once she develops a crush on Bloo she turns into a psychotic Yandere who continually tries to get rid of Bloo's best friend Mac so that she can have Bloo to herself.
Worst of all, Bloo barely even notices her and constantly forgets her name.
Where to even begin? History and the Evening News are bursting with stories like this.
Countless examples of someone discovering an infidelity and killing either their partner or their partner's lover or both.
There's also countless examples of men and women who have murdered the boyfriends, girlfriends, husbands, wives, crushes, what-have-you of the person they're interested in.
Cesare and Lucrezia Borgia were widely rumored to have been lovers in an example of Did They or Didn't They?? of epic proportions. Cesare was believed to be extremely jealous of Lucrezia's second husband, Alfonso of Aragon, both for Alfonso's good looks (Cesare's face had been scarred by syphilis) and because Lucrezia seemed genuinely happy with her husband. Cesare had Alfonso attacked by his men in the middle of the night, but Alfonso survived. His men retaliated by shooting arrows at Cesare one day while he was in his garden. Cesare shortly thereafter had Alfonso strangled in his recovery room. Lucrezia was said to be devastated.
It should be noted that Lucrezia and Alfonso's marriage was quickly becoming a useless alliance, which also probably had something to do with Cesare's motives.
Isabella of France and her lover Roger Mortimer led an invasion against Isabella's incompetent husband, Edward II of England, who was eventually forced to abdicate in favor of his and Isabella's son. When Edward was later murdered, Isabella and Mortimer were thought to be responsible. Once Isabella's son came of age and became King Edward III, he had Mortimer executed on fourteen counts of treason, including the murder of Edward's father.
While history books have long claimed that Mortimer and/or Isabella had Edward II murdered, more recent books, such as The Greatest Traitor by Ian Mortimer (No Relation) have argued that this was malicious gossip recorded by the writers of contemporary chronicles as fact. It has even been claimed that Edward never was murdered.
Henry Stuart aka Lord Darnley, the cousin and consort of Mary, Queen of Scots, thought that his wife was just too close to her secretary, David Rizzio. So he and other nobles interrupted a reunion between them and stabbed Rizzio 56 times in front of the pregnant Queen, despite her pleas and a failed Go Through Me.
Of course, depending on if you believe she had a hand in it, Mary might have done the same thing to Henry so she could marry Lord Bothwell.
Does the death of a career count? This happened with WWE Superstars Triple H and Chyna. Chyna was dating Triple H when she started in the company, and he was the start of her success. However, a few years later, Triple H starts dating Stephanie McMahon and suddenly Chyna is dropped back down into the Women's Division (Up until then, her entire gimmick was being a Amazonian Beauty who fought the male wrestlers) before getting fired completely. It's still unclear what happened here, either Stephanie didn't like her boyfriend's ex hanging around all the time, or Triple H (who is somewhat infamous for being an opportunistic ladder-climber) realized that Chyna had made him as big of a draw as she could, and simply threw her aside before moving on to Stephanie (who is head of Creative Writing and the boss's daughter.)
One episode of Deadly Women on Investigation Discovery featured a case of this where "Alice" was in on it. The woman had a pattern of getting married, meeting another man, convincing the other man to kill her current husband, marrying the other man, then meeting another man...What made the plan nearly foolproof was that the other man usually did all of the killing, and then the next man would kill that guy, so that there was virtually no proof of the previous crime.