"I feel like we hear the phrase "love and justice" a lot, but no one seems to ever mention "love and evil". An evil guy whose love is deeper than the ocean. If the object of affection is narrowed down to Celty, then I am one. If it's for the love between Celty and me, I'm pretty sure I can become as bad as bad can be. "
Onigumo from InuYasha was already pretty damn evil before he met Kikyou, but it was his desire for her that prompted him to sacrifice himself to a horde of youkai and become Naraku, the Big Bad of the series, who is much, much worse than Onigumo was as a human, considering the vestigial trait of Onigumo's POTENTIAL to feel guilt and empathy an unacceptable weakness to be purged from himself.
Mazinger Z: Dr. Hell fits the trope to the letter. He was psychologically unstable before... but when he fell in love with a cute Japanese student attended his college he became obsessive, jealous (he even fumed if she spent time with someone else) and erratic. When he found she was in love with someone else, his mind finally snapped out completely. He thought everyone was out to get him, and attempted to Murder the Hypotenuse (he stormed into the campus with a shotgun!). That incident (and another more where he tried to help someone and he got the crap beaten out of him for it) were his Start of Darkness. He dedided Humans Are Bastards and he would make everyone pay. People had shunned him out of scorn or indifference before, but from that day they gave him a wide berth out of fear because he already started looking Obviously Evil (and downright creepy).
Ranma One Half makes quite a heavy use of this, for a comedy series. Shampoo wants Ranma to return her love, so she tries to use Love Potions on him and has no qualms about contemplating Murder the Hypotenuse, though she only actually attacks her rivals if she's convinced she can do so without hurting her reputation. Ukyo Kuonji can be just as bad as Shampoo in the manga, but still keeps some trace of this in the anime. Mousse is the most common offender of attempted Murder the Hypotenuse due to his unrequited love for Shampoo. Ryoga Hibiki abuses his curse to take advantage of Akane Tendo's naivete due to his obsessive crush on her, at least once tried to use a Love Potion on her, once contemplated living together with her in a haunted cave forever so she would always want to cling to him for protection, and once tried to murder his rival for her hand — while his direct motivation was to make Ranma Saotome stop hitting on him (having accidentally got him with the aforementioned Love Potion), he openly recognized that killing Ranma would leave him with Akane by default.
Gendo Ikari from Neon Genesis Evangelion takes the phrase 'you're worth the world to me' to its (insanely) logical conclusion. He tries to bring about the apocalypse, in fervent belief that this will reunite his mind and soul with that of his wife, Yui. And it would actually have worked, if weren't because Yui's soul rejected Gendo's in the eleventh hour due to all the suffering he brought into the world in order to complete his plan.
Chikane Himemiya from Kannazuki no Miko does this to the point of violating her friend. This is all so that her friend will be willing to kill her. This is because she is guilty over killing that same friend in a past life where they were lovers.
Lelouch vi Britannia in Code Geass, to a certain extent. Lelouch seems to be rather affectionate towards his sister, to which many fans have joked that he has a sister complex. Lelouch often dotes on her and promises that he'll create a better world for her... which essentially means manipulating, blackmailing, sacrificing, and disposing whomever he wishes to ensure that his rebellion in Japan achieves a violent revolution against the Holy Britannian Empire.
Another more explicit example happens with Suzaku Kururugi. When he first appeared, he was a Knight in Shining Armor who later falls in love with princess Euphemia, but when she dies he pretty much ventures into Anti-Villain territory.
In R2. An episode after Lelouch and Shirley officially become boyfriend-girlfriend, she gets killed by Rolo. Lelouch then goes on a Roaring Rampage of Revenge against the Geass cult (which involved him slaughtering a lot of noncombatants) that sets off a series of events that leads to Zero's identity being exposed and him being betrayed/abandoned by the Black Knights.
Mao's psychotic devotion to C.C may be a more conventional case of Love Makes You Crazy, but if Nina counts as this trope, he would too.
And finally, this was the main reason Lelouch did not respond to Kallen in the last few episodes of R2 even after she kissed him, as he knew that if he told her he loved her (whether true or not) that she would turn against the Black Knights and the rest of the world to follow him. And by "follow him", I don't mean just supporting him in the coming battle; she would have willingly died with him. That said, this is more a case of "Love Makes You Support Evil", but it still counts.
In Gankutsuou, being on the losing end of a Love Triangle with his childhood bestfriends Mercedes and Edmond, causes Fernand's Start of Darkness, which he blamed in large part on the fact that he was still poor and struggling while Edmond was already rich and successful.
Yu-Gi-Oh!: While we're in that neck of the woods, we shouldn't forget Pegasus and his dead wife.
And of course Carly in 5D's. So evil that she claims that she became a Dark Signer just so she could kill Jack and they could be together. Thought technically, not so much evil as suffering from a bad case of Brainwashed and Crazy, coupled with a Superpowered Evil Side.
The most outright example, however, is Queen Beryl and her obsessive love for Mamoru aka Tuxedo Kamen aka Endymion in the first season. Fiore also goes the route of the villain to get at Mamoru in the R movie. Another one would be Katzy/Cooan in R and her own love for Rubeus. Also Zoicite, despite being already evil anyway, does most of his evil to please his boyfriend Kunzite, who then goes a little Ax-Crazy to avenge Zoi's death.
And in Pretty Guardian Sailor Moon, Sailor Moon loves Tuxedo Mask so much that she's willing to destroy the world for him. And does. Twice.
And that's not mentioning Prince Dimande's little obsession with Serenity. Of course, he ended up sacrificing himself to save her from Wiseman which may point out that underneath the whole trying-to-destroy-Earth-for-revenge thing, he's not all-together a terrible person... Sort of.
That's only in the anime. In the manga he tries to kill her.
A common theme in the second half of Mai-HiME, which claimed more than its fair share of characters:
Shiho (out of her love for Yuuichi and jealousy of Mai for what she thought was an attempt to take it away, uses her Child, Yatagarasu, to attack Mai several times, eventually forcing her into a battle in which Yuuichi will die if either wins. She is also at least partially responsible for the death of Akira's child, and thus, Mai's younger brother Takumi)
Shizuru (out of her love for Natsuki, goes insane when she believes Natsuki has rejected her (Natsuki, however, also considers Shizuru her most important person) and kills those she sees as Natsuki's enemies.)
Mikoto (out of her love for her brother, whom a few other characters eventually learn is the Big Bad, ends up becoming Brainwashed into doing his bidding).
In the manga, Shiho's desire to be with Yuuichi leads to her coming to the school as part of the Searrs takeover and fighting against Mai. Similarly, Alyssa's loyalty to her mother Saeko Kuga causes her to go along with her plans.
Nina in Mai-Otome She not only loves, but is in love with her adoptive father Sergay, who works for Nagi dai Artai. Seeing that Sergay is Arika's Anonymous Benefactor pushes her over the edge, unlocking the power of Nagi's Ultimate Black Diamond. She attacks Arika in a rage, but kills Erstin instead when Erstin has her Slave protect Arika. She works to further Nagi's plans, first to prevent him from punishing Sergay for withholding information and later uses the Harmonium in order to keep Sergay alive after Nagi shoots him, and is partly motivated by a belief that she can no longer turn back.
Also Tomoe, who is obsessively in love with Shizuru. She plots against Arika, trying to get her expelled, killed or otherwise unable to get between her and Shizuru, and later joins Nagi's Valkyrie unit so she can essentially have Shizuru for herself.
Shin from Hokuto No Ken kidnapped the protagonist's fiancée, Yuria, and became a wasteland overlord in his attempt to win her love.
Later in the same series, Souther is revealed to be enslaving children to build him a giant pyramid all for the sake of his beloved master, who he had been tricked into killing. In the end Ken declares that Souther was the one who loved the most of all.
Subaru from Tokyo Babylon and X1999 starts out as a sweet, kind, overly caring boy. Sometime later after the man he's in love with, Seishirou, destroys everything he cares culminating eventually with setting himself up to be killed by Subaru, Subaru decides to replace Seishirou as the Sakurazukamori assassin.
Arashi is an example of a well-meaning one, performing a Face-Heel Turn to save her love interest, Sorata, who's destined to sacrifice his life for her.
We also have Kanoe, who wants to free her older sister Hinoto from her obligations as the leaderess of the Dragons of Heaven, which give her enormous Psychic Powers but render her crippled, blind and deaf/mute. If she has to destroy the world to release the person she loves the most from such a cruel burden, she will.
The Twelve Kingdoms is a notable example in that there don't seem to be any romantic interests that actually end well (maybe because sex isn't a requirement for reproduction in that world?). UST throws jealousy fuel on the drama fire for the mains, and all the rulers shown so far that take consorts end up screwing up tremendously (and one queen's unrequited love for her partner ki-rin leads her to drive all women out of her kingdom).
Brain Powerd, one of the more recent Yoshiyuki Tomino anime, pulls this off not with romantic love, but with motherly love. Captain Anoha McCormick, leader of the Novice Noah finds out much to her dismay that her son Jonathan has joined the Reclaimers out of bitterness for feeling neglected by his mother as he was growing up. This leads her to fall into a deep depression over her failure as a parent, and to later actually join the Reclaimers herself under the alias of Baron Maximillian, using a Char Aznable-sque mask and a bulky body armor (not to mention a voice synthesizer) to hide her true identity. She then starts working for the same goal she had until then opposed: to guide Orphan through the atmosphere and into space in order to fulfill Jonathan's (and the Reclaimers') Social Darwinist wishes. She doesn't care that Orphan's emergence will cause The End of the World as We Know It, because she's now too obsessed with realizing her son's twisted ideals and somehow make up for her parental mistakes.
The sorceress Alcyone gave only one reason for betraying Cephiro: she loves Big Bad Zagato. Never mind that his actions could cause Cephiro's destruction. In the second season of the anime, she joins the newBig Bad to hasten Cephiro's destruction because Zagato rejected her. And he's dead, so she wants revenge. Although in season 2, she's mostly either Brainwashed and Crazy or half-mad with grief.
Zagato himself sent out several people to murder the Magic Knights, in a couple of cases taking advantage of said people's emotional vulnerability to manipulate them into evil acts and outright brainwashing one. He did this because he was in love with Emeraude and desperate to save her from her fate, which leads us to the third...
Emeraude's love for Zagato made her unable to support the land as Pillar, which is the real reason it's in such a bad state. When the Magic Knights kill Zagato, she completely loses it and tries to get revenge. Her fear of just such an event is why she summoned the Magic Knights in the first place: to kill her before she could destroy Cephiro with her power.
One of Patrick Zala's main motivations in Gundam SEED was to avenge the death of his wife Lenore, killed by Naturals during the Bloody Valentine Incident.
Subverted(ish?) in Ask Dr. Rin!. Tokiwa appears to be obsessed with becoming Meirin's boyfriend, but he's possessed, and the demon has more sinister motives. However, he seems to have feelings for her even afterward, and the series never made clear exactly how he got possessed in the first place, so it's at least possible, if not likely, that the two are connected.
The Yandere Yuno from Mirai Nikki tries to justify her evil deeds and homicidal tendencies by saying it's all for Yukiteru's sake and it also corrupts Yukiteru.
In the GSC Arc of the Pokemon Special manga, the Mask of Ice, later revealed to be Pryce, the Ice-type Gym Leader of Mahogany Town. His motivation behind resurrecting Team Rocket was out of love for his Lapras. He wanted to capture Celebi for turning back time so his Lapras could return to its parents. Even at the cost of the world freezing over from turning back time.
This is Ren Sohma's Freudian Excuse in Fruits Basket. She was the maid who took care of the former leader of the Sohmas, Ill Boy Akira, and the only one able to reach for him. But when Ren married him, got pregnant and started fearing that he'd love her less once their child was born... Ooooh, boy.
Misa Amane from Death Note. She loves Light Yagami because he is Kira, and then she loves Kira because she feels she's in debt to him for killing the burglar who murdered her parents when Misa was young. And because she loves Light... Misa becomes the Second Kira.
Fei Wong Reed from Tsubasa Reservoir Chronicle is all but outright stated to be this. He claims the purpose of his plans is to prove he's the most powerful magician of all time, but some of his words later seem to suggest otherwise. He enacts a centuries long unimaginably complex Gambit Roulette that involves the death of millions (if not billions- at least one UNIVERSE has ended by Ass-chin's deeds), all for the sake of bringing someone (Yuuko) back to life. Or keeping her from dying. Un-dying? Un-unkilling her? Terminology gets really confused at this point, as does the reader. She died years ago, but a certainReality Warper with Power Incontinence accidentally caused the universe to not notice it; Fei Wong wants to either keep this state of affairs or make her 100% alive rather than zombie-ish. Eventually, it's revealed that Fei Wang Reed was likely to be the "afterimage of a wish made by someone with strong magic", meaning that he was probably created out of the said Reality Warper's deep subconscious desire for Yuuko to stay alive, ie he was the physical manifestation of "Love Makes You Evil"
Naturally, she makes reality retroactively notice her death as payment to help to stop him.
Wendy from the Read or Die the TV, who sheds her clumsy demeanor to becomes a cold advisor all out of admiration for Joker.
A recurring theme in the current arc of Naruto, is that love (not exclusively romantic love) can lead to hatred and war. This is seen in Sasuke wanting to destroy Konoha in order to avenge Itachi and Pain's Start of Darkness.
As quoted by Tobi or Uchiha Madara or someone else:
"When a man learns to love, he must bear the risk of hatred."
This becomes more meaningful during the chapter that revealed that Tobi was Obito. The chapter is basically a long flashback that shows Obito's childhood, focusing on, rather than his relationship with Kakashi, his relationship with Rin, the girl who he was in love with. It has now been confirmed that Rin is dead (at the apparent hands of Kakashi, none the less) and that her death is what caused Obito to become Tobi. Essentially the entire basis of his plan is to create a world where Rin is alive.
It is ultimately revealed that this is the nature of the Uchiha clan's curse. Their love is stronger than that of any other clan. However, when an Uchiha loses someone important to them, that love causes a special reaction in their brain that drives them to madness and empowers them with The Power of Hate.
In Seton Dobutsuki Risu no Banner, most of the villainous, bowtie-wearing Spitzohr's dirty doings revolve around attempting to steal away Sue, whom he loves. (This includes getting the hero doped out of his mind on shrooms in one episode.)
In Spiral, Kanone wants to kill all of the Blade Children... because they're basically cursed abominations not meant to exist blablabla, yes, but also because "all of the Blade Children" includes his best friends (and himself) and he really thinks it's for their own good, just so they don't suffer more than necessary, don't fall into despair like he has, don't lose their selves (a very real Threat), etc. This leads to many breakdowns of the "Why don't you understand me ?!" kind.
In Prétear, a Magical Girl named Takako falls in love with Hayate, but he rejects her. In her despair, she calls the forces of darkness to her and becomes the Princess of Disaster. And in the anime, when the Knight Sasame realizes that nothing he can say or do will convince Takako to become good again, he reveals that he's always loved her, and betrays the Knights so he can stay by her side (even though he's fully aware she doesn't love him).
From Bleach, we have Tousen, whose close friend was killed by her shinigami husband, after she had scolded him for killing his comrade in battle. So what he does is join up with Aizen, pretty much wants all the shinigami dead and is willing to destroy a town full of people, all to get revenge on every single shinigami. Talk about Disproportionate Retribution.
Jesus, Gin. In chapter 415 we finally learn that he allied himself with Aizen, thus taking part in many murders and betraying Soul Society, only to find the right moment to kill Aizen for attacking and stealing a part of his childhood friend Rangiku's soul when she and Gin were still living in Rukongai. Talk about going to great lengths.
Vincent from Pandora Hearts will do anything, anything for his brother Gilbert, whether Gilbert likes it or not (almost always "not"). Gilbert himself is at least sometimes in the Love Makes You Crazy category for Oz. It hasn't been revealed how far he'll actually go for Oz.
Glen is shown in flashbacks to have these tendencies. At least until it's revealed that Jack, not Glen, was driven insane by his love for Lacie and by her inevitable death. Oswald-Glen actually handled Lacie's (who is actually his younger sister) death fairly well for someone forced to kill his only family.
Rozen Maiden has both Suigintou and Kirakishou subject to this, although Suigintou's case is not as severe as Kirakishou's. Still, Suigintou loves her father so much that she is willing to discard almost all signs of friendship just to meet father and become Alice, partially due to a sense of incompleteness and personal betrayal at the hands of Shinku. Kirakishou, however, takes it a few steps further and imprisons past and present mediums of the other dolls in crystal, having some kind of love-crazy obsession over both the mediums and the dolls. So much she wants to eat her sisters, to make them part of her. Interestingly enough, Suiseiseki, who is also motivated by love, manages to avoid this trope completely, and can arguably be classified as the most caring doll in the series, despite her habits.
With its various murders and other crimes over the year, it's only natural that Detective Conan should visit this trope from time to time. This includes cases where the victim's obsessions with the person they loved led to blackmail, and eventually their murder. And in one case a man became so obsessed with getting one woman to love him that he drugged her and set her in a burning building planning to 'rescue' her and become her Knight in Shining Armor. It's not only related to romance, either. There's also:
Sibling love (a Buddhist novice kills his master after learning he murdered his older brother, an average guy becomes a thief to force the police investigate his younger sister's presumed murder, a ghost writer brutally kills his boss after he causes the death of his Ill Girl sister when he was supposed to pay for her treatment, an Innocent Flower Girl brews a Thanatos Gambit to punish her older sister's rival who stole her tricks and drove her to kill herself; an uni student enters a local Tokusatsu fan club and kills the biggest Otaku in the group for stealing his little brother's memorabilia out in the open and cauing the kid to get hit by a truck;
Parental love (a widow keeps her son locked up for years because she doesn't want him to turn himself over after accidentally killing his dad during a fight, a businessman stages a Batman Gambit to catch and kill the hitman who caused his little daughter's death by proxy during one of his "missions"; a security guard beats up ganguro girls after one of them accidentally runs over his son);
Friendship (a computer expert disguised as a Corrupt Corporate Executive's butler kills his boss to avenge his best friend, driven to bankruptcy and suicide by him; a teen detective kills another fellow sleuth after he wrongfully accuses her best friend of killing her boss and causes her to kill herself too);
A mentor/teacher's love for their pupils (a school teacher kills two of her co-workers after they murder one of their students when she finds out they're involved in money frauds; an uni professor kidnaps Takagi and puts him in a cruel Death Trap while forcing half the Beika police force to watch because he mistakenly believes that Takagi is the man who dumped his favorite pupil and caused her to go the Spurned Into Suicide way.)
The yaoi manga Under Grand Hotel has a few examples. 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 screams at him, "Get off of Sen! He belongs to me!" Immediately after that Lain is shot and killed by security guards.
Also, Swordfish acts in such a manner whenever Sen is involved. He 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. He also nearly raped Sen to almost death in one scene.
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.
In the Trigun manga, Hoppered the Gauntlet joins the Gung-Ho Guns so that he may get revenge on Vash, whom he believes destroyed July City and the blind girl Hoppered looked after who lived there. Also, Leonof the Puppet Master may have run away from home to become a Gun after being rejected by a woman he loved.
Near the end of Mirumo De Pon, it's revealed this is the Big Bad 's Freudian Excusefor turning toThe Dark Side and wanting to destroy Mirumo and the Fairies' royal line: in his younger days, he got cruelly rejected by the girl he loved. Mirumo is not pleased when hearing this and chews Dark out a new one, calling him an idiot and a pain for such a ridiculous motive.
Marco, Manipulative Bastard par excellence and quarterback/quarterback spy for the Hakushuu Dinosaurs in Eyeshield 21 is motivated entirely by his desire to impress his crush—and upperclasswoman—Maria, manager of the team. To do so, he institutes a regime of brutal plays, using Gaou and Kisiragi to cripple the quarterbacks and recievers of other teams. In the process, of course, he only drives Maria further away. Interestingly, she may have confessed her feelings to him after his final defeat.
Shinra of Durarara!!, despite being an otherwise Nice Guy, has explicitly identified himself as stated in the quote at the top of this page.
Enchu worked hard to become an executor so that he could support his sick mother. His Start of Darkness was his mother dying of illness around the same time that he lost the position to Muhyo. Several characters notice that he's a potential victim for this trope, as Page tells him that one of the reasons he was passed over was his preoccupation, and Teeki caused his mother's death so that he could eventually take his body.
The Cortlaw siblings- Kid, Ivy and Mick- want to bring their parents back from the dead.
Panza became attracted to Roji after he showed her kindness by giving her a spare application form, and was thus desperate to be noticed by him, enabling Teeki to manipulate her into joining Ark.
The haunt that Muhyo and Goryo are competing to exorcise is that of a single mother who went insane after her daughter was killed in a traffic accident, obsessively creating the dolls that her daughter wanted as a gift (which her haunt form uses against the heroes). Eventually, the spirit of the daughter intercedes, and while the daughter would have gone to Heaven while the mother went to Hell, both end up in the Styx (essentially Purgatory) by way of a compromise
SD Gundam Force has this trope by way of Deathscythe's Start of Darkness. His obsessive love for Lacroa's Princess Rele is what led him to join the Dark Axis, hoping to hijack the kingdom's magic and use it to become human like her.
An oddly literal example from Princess Tutu: the fragment of Mytho's heart that contains Love gets soaked in Raven's blood, so that it turns him evil when it's returned to him. Also, in a more traditional example, Kraehe's motivation turns out to be her desperate desire for his love.
Rune of Karakuridouji Ultimo nearly rapes Yamato when his feelings for him go unreturned. As well as the other evil things he does out of love for Yamato.
This is the reason for everything that goes wrong in Wolfs Rain.
Attack on Titan takes place in a Crapsack World full of grey morality, so it's hard to say it's a negative thing, but Mikasa loves Eren, and there is no one and nothing she wouldn't kill to protect him, and the same goes for Armin (though he's not the type to fling himself facefirst into trouble like Eren is). Getting on Mikasa's bad side is about as good an idea as jumping off a cliff. Annie finds that out the hard way.
In chapter 47, Ymir admits that her love for Christa is wholly selfish. Ymir is willing to endanger Christa just for the chance to see her again and outright kidnaps her because she's "a shitty human being".
Just to be clear Homura doesn't want Madoka all to herself; in fact she doesn't even care if Madoka loves her back. Rather, Homura just wants to see Madoka happy...and because she loves Madoka so much, she's willing to go to any lengths to do so.
Black Adam in The DCU first turned to totalitarianism when his wife was killed while he was away. Eventually (as in thousands of years later) he started to reform, especially when he fell in love again with Isis as seen during 52. When Isis was murdered, he leveled an entire nation and took on every active superhero on Earth at once.
Later he combined this trope and With Great Power Comes Great Insanity. When he resurrected Isis with a fraction of his power she turned evil, ripped Captain Marvel from his powers and wrecked a chaos in the world seeking revenge. Later in that same story Mary Marvel, corrupted with powers Black Adam gave her, threaten Billy's ex-girlfriend, forcing him to take a part of her powers, which turned him evil.
Harley Quinn in the DCU. She was a psychiatrist at Arkham asylum until she was assigned to the Joker. Falling in love with him, she broke him out and joined him as his girl sidekick. To what degree the Joker returns her feelings is questionable, ranging from 1% to zero. However, almost all her evil acts are an attempt to secure his affection.
Also, in some versions, this is why Mr Freeze is a villain, for the love of his Ill Girl wife Nora.
And according to The Killing Joke, The Joker himself would not be who he is if he did not (once upon a time) have good intentions to give his late, pregnant wife a better life.note No, she was not late when he was trying to do so. Her death is part of what drove him to utter madness. This, however, is probably a case of Multiple-Choice Past.
Ramona Flowers of Scott Pilgrim has seven evil exes that the titular character must defeat, and in a few cases the ex-boyfriends have been hinted at as being not exactly evil to begin with, but turning evil after the inevitable breakup. The same fate may befall Scott, as well.
Depending on the writer, Marvel Comics villain Whirlwind is this. At one point, he accuses the Wasp of "owing" him, because his attacks on The Avengers were attempts to get her attention.
The Jackal's Start of Darkness began with the death of Gwen Stacy. As Professor Miles Warren, he was secretly in love with his student, and could never get it out of his head that Spider-Man (who he later discovered was another of his students and her boyfriend) was more to blame than the Green Goblin was. (Why he would actually work for Norman Osborn, the true mastermind behind the Clone Saga, is anyone's guess.) Indeed, he tried to recreate her with a clone of Gwen (dozens, actually, but only one shared Ben Reiley's apparent immunity to the degeneration factor). When the second Carrion (Malcolm McBride) appeared (with Warren still believed dead) Spidey called Warren "A sick man obsessed with a dead woman" in disgust. But even when revealed to be alive, he would never let Spidey forget it. In fact, in one of the last fights between them (when his consciousness was inside the body of a coroner who had turned himself into the third Carrion, in Spider-Man: Dead Man's Hand), Spidey told him, "Still hung up on Gwen, huh? Some things never change."
Skurge the Executioner fought on the side of evil because he was in love with Amora the Enchantress. At first it seemed like she only strung him along, using him as valuable muscle, since he was strong enough to fight Thor. But she actually did miss him and even tried to bring him back from the dead several times.
Still in the same universe, a juvenile, nicer incarnation of Loki gleefully explain to his current love interest that he was willing to postpone whatever plan he had to help Asgardia, thus risking it failing, just to increase the odds of saving her too
In the Squadron Supreme limited series, Golden Archer brainwashes his girlfriend after she rejects his marriage proposal.
Hyperion's extradimensional duplicatemurdered Zarda's husband after falling for her. Admittedly, this is more accurately an example of "Love Makes You More Evil Than You Were Already".
At some point in time during The Ballad Of Halo Jones, Toby the robot guard dog brutally murders his owners in secret. When Halo listens to his old memory tapes and hears her friends dying, she asks herself why he did it. Whereupon he promptly appears in the room and says "Love, Halo. I did it all for love." Toby's owners had left him to Halo in her will and apparently the robot dog's love for Halo was powerful enough to override any programming he might have along the lines of "Hey, robot dog, don't kill your owners." Cos Halo is just that sexy, obviously. Anyway, Halo pretends to be pleased that he loves her that much, but Toby can tell from her heartbeat that she's hell terrified, so he attempts to kill her. And fails, of course.
Jean Loring's ultimate motivation in Identity Crisis. The "so we can be together" quote in the trope description is even repeated, almost verbatim, at The Reveal.
Becomes a Discussed Trope in Archie Comics' Mega Man. Agent Stern warns Dr. Light that giving his robots emotions is a really dumb idea because of this trope and gives some nice Foreshadowing of the Mega Man X series in the process.
Koga from The Sun Soul qualifies for this trope. He might have betrayed his clan, city, and code to Team Rocket, but only because he wanted to take care of his daughter.
In Peggy SueFix FicThe Second Try, Shinji caused Third Impact because Asuka rejected him ("No one loves me, so everyone must die"). In the altered timeline, he almost caused the Third Impact again because he believed Asuka was dead.
Yui: Is it okay now? Shinji: Yeah. She's fine. That's all that matters to me.
In RE-TAKE: Shinji, very nearly — "I will throw the world into the pits of hell to find Asuka again!"
Inner Demons: It's implied throughout the story (and eventually confirmed posthumously at the end) that Trixie is in love with Twilight Sparkle. That's why she ignores her moral compass and becomes The Dragon when Twilight is consumed by the Darkness and becomes the Big Bad.
The Ironic Echois Beth, who, having previously dressed Holmes down for willingly surrendering, does so herself, trading her life for his, Watson's, and the Doctor's. Also, like Holmes, her motivations are not entirely pure: part of her bargain is that she be kept in Victorian London and not have to leave that era (and, by extension, Holmes).
While Vanitas from The Shrouded Path certainly isn't a good guy, he does seem to have some morals. Then he gets a chance to finally be with Aqua and Terra and all those go out the window.
Likewise, many of Ven and Terra's morally ambiguous actions, especially in the sequel, are motivated by their need to protect each other and Aqua.
Once-noble Jedi Anakin Skywalker from Star Wars became the Sith Lord Darth Vader because he hoped the Dark Side of the Force would grant him the power to save his pregnant wife from the death by childbirth he foresaw. What's interesting is that, three films later, his son's love for him made Anakin good again.
When Sarah finally confronts Jareth at the end of Labyrinth, he claims that he's put her through all this because he loves her. Whether he is sincere, taking a desperate gambit to delay her until time runs out, or trying to tempt her to The Dark Side is an open debate. Most of the Fanfic community (naturally) falls into the first camp or a combination of the three.
The version of Dracula in Bram Stokers Dracula declares war on God because of love (specifically, because his fiancee killed herself upon hearing lying reports of his death, and the church taught that her act irredeemably damned her soul to Hell).
The 1956 horror film The Black Sleep stars Basil Rathbone as a 19th century neurosurgeon named Cadman who uses experimental brain operations to turn his colleague into a mindless brute with an incestuous longing for his daughter and keeps him around to force his daughter to work for him, has his assistant abduct homeless people with promises of shelter and pay, destroys a test subject's ocular faculties so he cannot see and escape, frames his star pupil for murder, fakes his pupil's death, and then blackmails him into helping him or risk exposure. Why is Cadman doing this? So he can learn enough about how the brain functions to save his wife from a paralyzing brain tumor.
Imhotep in The Mummy Trilogy. He spends most of the two movies he's in trying to bring Anck-Su-Namun back. And, you know, raining down the ten plagues of Egypt and sucking the life out of anyone who stands in his way. He's a combination of this trope and Cursed with Awesome, with some Bad Powers, Bad People thrown in for good measure (that is, even if you weren't so evil to begin with, becoming able to rain down the ten plagues of Egypt is a sure sign of villainy). In a rather brutal twist, it turns out that all of Imhotep's actions were in vain. He finds out in the last moments of his life that Anck-Su-Namun doesn't love him as much as she loves her own life, and she certainly doesn't love him as much as Eve loves Rick.
Davy Jones in the Pirates of the Caribbean sequels. He fell in love with Calypso, goddess of the Sea, and became Captain of the Flying Dutchman so they could be together in ten years. When she failed to show up, Jones carved out his own heart, helped the Pirate Court trap Calypso in a mortal body, and made the Dutchman his personal torture chamber, (turning himself and its crew into Fish People as a side-effect).
Auntie in the Japanese campy slapstick horror-comedy movie House (Hausu) kills and eats young, unmarried girls and possesses the body of her niece just so she can remain immortal and marry her long dead fiancee, who she believes will return to her.
The Hole combines this with Love Makes You Crazy in not just one, but two examples, owing to its use of The Rashomon. In the first version of the story, Liz explains that Martin's love of Liz and jealousy of her liking Mike led him to lock Liz, Mike, Frankie and Geoff in the titular bomb shelter (in which they had been hiding to avoid going on a school trip) until she realized she didn't really like him very much. In the second, true version, it is revealed that Mike wanted to leave the bomb shelter early to go patch things up with his ex, but Liz had become so obsessed with him that she decided to lock herself and the others inside the bomb shelter to prevent him from leaving, ultimately leading to Frankie, Geoff and Mike dying. She then kills Martin too to cover her tracks.
The loss of his wife is what primarily pushes Khan over the edge in Star Trek II. Not that Khan was ever a nice guy, of course, but beforehand he was an apparently sane Social Darwinist, as seen in "Space Seed". Nero has similar issues in the new film due to his own dead wife.
In Hot Fuzz Frank Butterman forms a deadly city council to eliminate all flaws to the community after his wife commits suicide after Travelers ruin the town, causing them to lose the Village Of The Year award. Lampshaded by his son: "If Mum knew what you were doing she'd kill herself again!"
Lots-O'-Huggin' Bear from Toy Story 3. After believing he was spurned by his owner (who loved him more than anything in the world), he becomes a twisted tyrant of the daycare center and a G-rated nihilist.
If you assume that this trope could come up in a film called Love is the Devil, you are most certainly right in doing so.
It appears that this is Horvath's true reason for betraying Merlin and siding with Morgana in her quest to destroy the world in The Sorcerer's Apprentice. He just couldn't handle Veronica choosing his best friend Balthazar over him.
In The Last Circus it is Javier's love for Natalia that leads into his downfall and eventual descent into insanity.
Non-romantic love, but Loki in Thor loves his father and his Asgard home very, very much. His desire to be acknowledged and loved on par to his brother Thor leads him to attempt genocide.
In The Other Boleyn Girl Henry tells Anne how he broke off from the Catholic church for her. "I have torn apart this country for you, gone against every principle in my heart, I've endured the doubts and counsel of good men and silenced them with the axe...just to be with you."
In the 1960 French horror film Eyes Without a Face, Dr. Génessier's beloved daughter Christiane is in a car accident that leaves her with a horribly disfigured face. Obsessed with making her beautiful again, the doting father starts kidnapping other young women and transplanting their faces onto his daughter's.
In X-Men Origins: Wolverine, Victor did not take his brother James leaving him for a life of peace well... not in the least.
Cypher/Mao Fanchu/Wu Zhang's motivation in the Hive Series is stopping Overlord because of the murder of his wife. He even goes to the lengths of attempting to kill thousands of HIVE students.
The Silmarillion: Maeglin of Gondolin lusts after his cousin, Idril. Since he can't have her, Maeglin ends up willingly betraying Gondolin's position to Morgoth. Notable in that Maeglin was the only elf to ever be sexually perverted. Ever.
Celegorm's (implied) planned rape of Lúthien was explained by "love" in the book, although it is debatable whether one may call that "love". "Lust" seems more appropriate.
One of the main themes of A Song of Ice and Fire is that love isn't much of a motivation for good. Jaime Lannister, Petyr "Littlefinger" Baelish, Lysa Arryn all did some pretty awful things which they claim was motivated by love. It's also at the root of the original civil war against the Targaryens.
It's interesting to note that in the case of Jaime, as soon as his near-worship of Cersei begins to wane, his Heel-Face Turn begins.
It's difficult to get a solid reading on the motives of Petyr "Littlefinger" Baelish because of all the effort to which he goes to conceal his motives from rival players of the game of thrones, but the going interpretation from what little he's let slip is that he single-handedly instigated a massive civil war, resulting in the suffering and death of millions, just so he could get the husband of his childhood sweetheart disinherited, banished, and (most importantly) get their marriage invalidated. He could then pick her up on the rebound after having gathered enough power and wealth on the side to make a socially acceptable marriage proposal to a Noblewoman of her status. Or, in short, Murder the Hypotenuse turned Up to Eleven. It doesn't end very well for him though, as she also gets killed during the war. He then chooses her daughter instead.
Surprisingly, Tywin Lannister may be a partial example (more like Loves Makes You Eviller). He was never a nice guy, but it seems that his wife's death was a Despair Event Horizon which diminished his ability to care even for his children.
Skulduggery Pleasant: Caelan justifies his murder sprees as this, claiming that the women he stalked and obsessed over seduced him, broke his heart and drove him to violently murder them, it was even his Motive Rant while he was choking Valkyrie, but it's very likely that his "reasons" mostly exist in his head, and he's truly just a severely screwed up individual.
Eponine of Victor Hugo's 1862 novel Les Misérables provided a relatively mild example of this: in love with Marius, the quite literal boy next door, and aware that he was besotted with Cosette, Eponine disguised herself as a boy and lured Marius to the barricade, intending for them to die together. She eventually took a bullet intended for him and died giving him Cosette's letter, though to this day her motives are debated. Considering that Hugo's own daughter Adele became utterly obsessed with a soldier, running away, printing fake marriage announcements, and even trying to drag him to a hypnotist so that he'd marry her, she may have inspired some Truth in Literature on the part of her father.
In Hugo's The Hunchback of Notre Dame, it is only once Claude Frollo falls in lust/love with Esmeralda that the nastier sides of his personality come out. Pretty much every instance of death and disaster in the book occurs as a direct result of his obsession.
Inverted with the Big Bad of the series, Lord Voldemort. A running theme in the books is that his evil stems in part from the fact that he can't understand love.
It's possible that Dumbledore had a crush on Grindelwald that caused them to spend time together, and that he influenced Dumbledore into becoming the Muggle-hating bastard he was until his Cynicism Catalyst prompted his Heel-Face Turn.
"Kissin' Kate Barlow" from Holes, driven by grief over the loss of the man she loved, becomes an infamous outlaw. Previously, she was innocent Katherine Barlow who made delicious peach preserves and taught schoolchildren.
The Reynard Cycle: Reynard's dogged pursuit of the Countess Persephone leads him to sacrifice the lives of several of his companions and murder enemies both real and imagined. When it all turns out to be for naught, things take a turn for the worse.
The Rifter: A "Grief Makes You Evil" variant. When Ravishan died, John sank all of northern Basawar in the sea. But then he pulled himself together and began to rebuild the world. Laurie, however, mourning for her husband and child and ruined life, became a true monster, and never recovered. Love on the other hand is a great force for good.
C. S. Lewis believed that, without divine grace, human love is ultimately selfish and eventually destroys the object of its affection. In other words, Divine Love Redeems, but Human Love Makes You Evil. This plays out in a few of his novels:
In Till We Have Faces, Orual's love for her sister Istra leads her to unintentionally cause Istra to be exiled. At the end of the novel, Orual realizes to her horror how much of her "love" was actually jealousy of her sister and the gods. Istra and the gods forgive her.
In The Great Divorce, it's not uncommon to hear damned souls demanding that those they love in Heaven accompany them back to Hell. What makes it worse is that, in this story, there is nothing — besides their own Pride — preventing the damned from being redeemed and entering Heaven themselves.
This, however, is not love, as Lewis makes anviliciously clear via his Author Avatar: Excess of love did ye say? There was no excess, there was defect. She loved her son too little, not too much. If she had loved him more there'd be no difficulty. I do not know how her affair will end. But it may well be that at this moment she's demanding to have him down with her in Hell. That kind is sometimes perfectly ready to plunge the soul they say they love in endless misery if only they can still in some fashion possess it.
In The Four Loves, discussing three out of four of the lives goes into how they go evil.
Agatha Christieloved this trope. Many of her characters' motivations for murder came out of love for their significant other. Notable examples include Vera Claythorne from And Then There Were None (although she wasn't the murderer, her motivation for her crime, allowing a child in her care to swim out to sea and drown, was for her lover, who also happened to be the child's uncle, to inherit his estate), Jacqueline de Bellefort in Death on the Nile, and Richard Symmington from The Moving Finger. In Nemesis, the girl whose murder instigates the plot is said to have died because of "love, [...] one of the most frightening words there is in the world." Her adoptive mother, rather than give her up so she could marry a delinquent, gave her an overdose of sleeping medicine, secretly buried her in the garden, then strangled and bashed another girl's face in so everyone would think the delinquent had killed her.
Scrooge's greed in A Christmas Carol ultimately stems from his desire to marry and provide for his impoverished childhood sweetheart, who has no dowry. Unfortunately, the turn his personality takes in scrabbling to be wealthy kills Belle's love for him.
One of the major themes in KJ Parker's Engineer Trilogy. After being condemned to death by the Mezentine Empire, Ziani Vaatzes kills three people with his own hands, escapes to a technologically backward country and directly causes the deaths of over seventeen thousand Mezentine soldiers by overseeing an advanced weaponry program, all the while intending to betray the people he's selling the weapons to, because, in his own words, "I'm in love with my wife, you see. If I die, I'll never see her again. So I had to live. It's that simple." It's made all the worse by the fact that Vaatzes knows exactly what the consequences of his actions are going to be and plans them meticulously in advance...but never thinks of himself as having a choice in the matter, because love is making him do it.
Ashfur in Warrior Cats He nearly murders his former Love Interest Squirrelflight and her sister's children by blocking their escape from a raging fire, all so that she could feel the pain he had when she rejected him. And this is after he attempted to murder her father and get her mate framed for it, for the same reason.
Mapleshade, also from Warrior Cats, could also count for this trope.
Show Within a Show example: In The Mirador, Mehitabel stars in a play where the villain commits multiple murders to win the heroine's love and tries to justify it by telling her it was "all for you".
Hester Shaw in Predator's Gold. She sells Anchorage's location to the Huntsmen of Arkangel, out of jealousy for Freya, who she thinks is trying to steal Tom from her.
In Juliet Marillier's Son of the Shadows, the heroine's rejection of a suitor makes him more and more evil, culminating in several attempts to kill her lover, among other things.
Kirsten in Deltora Quest 3: Shadowgate. She fell in love with Bede, a Masked One, who seems to reciprocate.. .but he fell for her sister instead. Her response involved yandere-like jealousy, using dark magic to enslave Bede to her, transforming her sister into a pendant to control him and turning him into a decoy guardian to confuse anyone seeking to destroy the Sister of the North.
K.J Parker's The Scavenger Trilogy: Several characters cause great harm under the influence. Ciartan's obsession with Xipho leads him to kill her unborn child. His love of Xipho and his schoolmates leads to his pivotal betrayal of Tazenticus. Lysalis seems hollowed out by it, and is capable of doing anything in its name, as would her father.
This could be a subtitle of Wuthering Heights, although it would tie with Love Makes You Crazy. This obsessive, unhealthy love and infatuation between Catherine Earnshaw-Linton and Heathcliff contrasts with the later romance between Catherine Linton and Hareton Earnshaw.
Great Expectations: When Miss Havisham falls in love with Compeyson, who takes her money and then leaves her (he was a swindler and a thief), she decides to use her beautiful adopted daughter Estella to wreak vengeance on men. She eventually sees the error of her ways, which leads to an My God, What Have I Done? moment before she meets her end.
Definitely the case for Vlad Dracula and Elizabeth Bathory in Count and Countess. To get to Elizabeth's time period, Vlad has to find a way to outlive his natural lifespan. For that cause, he will do anything. Even if it turns his entire nation against him.
The Tortall Universe's Provost's Dog (Beka Cooper) series has Tunstall, who joined the dark side in a misguided attempt to secure a future for the woman he loved, Sabine of Macayhill.
Star Trek Expanded Universe novel Reunion (featuring characters from the later Star Trek: Stargazer series, although taking place later in the timeline) involves an assassin attempting to kill several characters, while framing Idun Asmund (who was raised by Klingons) of the crimes. In the end, it is revealed that the culprit is Dr. Carter Greyhorse, who blames the characters for the death (accidental) of Gerda Asmund, Idun's twin sister, with whom he was in a secret relationship for 18 years.
Philip Cutler from the Cutler Family series by V. C. Andrews originally dated the titular heroine Dawn in the first book. But even when it's revealed that Dawn is in fact Philip's sister/half-sister/aunt and she loses all romantic interest in him, Philip's original interest becomes exceedingly twisted and he still lusts after her regardless of their blood relation and eventually rapes her. He holds onto his obsession well into adulthood and eventually marries a woman he attempts to mold into Dawn's image. When Dawn dies in Midnight Whispers Philip transfers his obsession to her daughter Christie.
"Love doesn't lead to the dark side. Passion can lead to rage and fear, and can be controlled, but passion is not the same thing as love. Controlling your passions while being in love, that's what they should teach you to beware, but love itself will save you, not condemn you."
There's even more to it than that. Willow goes Psycho Lesbian, sure, but it's a personal vengeance thing, until she absorbs Giles borrowed magic power, at which point she feels all the pain and suffering in the world and decides to put everyone out of their misery.
And how can we forget Buffy's first boyfriend, Angel, for whom it was quite literally true? After experiencing "perfect love and happiness" in Buffy's arms, her beau reverted to his evil, soulless counterpart Angelus.
Even Spike could fit this trope somewhat. While he was first evil, he slowly fell in love with Buffy and more or less turned good by fighting at her side. When Buffy and him began a purely-sexual relationship, Spike did not stop bothering Buffy to make her to upgrade their relationship. She constantly refused and, when she finally put a stop to it, he turned back into his evil and tried to rape her to show her that she really loved him! Her cutting remark ("Ask me again why I could never love you!") prompts him to switch back to the good side after, but it took defeating demons in Africa to get his soul back and earn a chance at a real Heel-Face Turn based on something other than getting in Buffy's pants.
In one episode bewitched girls consider killing Buffy so they can get together with Xander... yeah magic.
The argument could definitely be made that Faith fits into this category as well, particularly if one believes - as many do - that the subtext throughout her arc indicated that she had feelings for Buffy . This would strengthen the argument, but even if restricted to purely platonic love, here is this; all Faith ever wanted was to be loved, by the Scoobies, but particularly by Buffy, as is confirmed by the First in Season 7. How does she react when she thinks that she has essentially been rejected by them all? She goes evil. Granted this is not the only force at work here, but the change is at least in part driven by her desire for love.
It's very heavily implied that Andrew had a crush on Warren. Practically stated.
Michael in LOST murdered Ana Lucia and Libby and sold out his friends to the Others to save his beloved son Walt, kept as a hostage by said Others.
Ianto Jones in Torchwood tricked his boss into giving him a job so he could hook up his potentially murderous girlfriend in the basement and hopefully revert her to human form, and even after she tried killing him, he tried to defend her. She could have killed the planet - he saw what happened with the other Cybermen.
When the power to destroy worlds falls into the hands of a small child who wants his mother. As the Doctor puts it, "There isn't a little boy born who wouldn't tear the world apart to save his mummy. And this little boy can."
River loves the Doctor so much that when she's being forced to kill him during a fixed point in time, she refuses, causing time to collide with itself and begin disintegrating. Normally, this would only be a matter of Love Makes You Crazy, but when she finds out what she's done, she not only continues, but claims she'll suffer more than anyone else in the universe if she has to kill the Doctor.
In Babylon 5, Lennier's last act in the series is an attempt to kill Sheridan so Lennier can have Delenn for himself. It goes badly...
John Sheridan narrowly avoids this trope... TWICE. When he first sees Mr. Morden come on the station, knowing he had been on the same ship as his wife Anna when it exploded, he has him arrested and held without cause or proof. The act is so much against Garibaldi's principles (who has been known to bend the rules a few times) that he quits. It is only when he is told that there is a greater danger that he snaps back out of his obsession and frees him. The second time, he witnesses Delenn's stabbing and runs down the sociopathic would-be assassin and very nearly kills him... with his bare fists.
Given that the latter assassin is a guerrilla under the command of a government Sheridan is at war with, killing him would be perfectly justified.
Dangerously Genre Savvy Morden uses this against Londo by murdering his true love Lady Adira and implying that it was done by Lord Refa, Londo's rival:
Londo: "All I want now is revenge and safety for my people. The rest of the galaxy can BURN for all I care."
Depending on how one interprets Delenn's connection with Dukhat, this trope might apply here as well.
Subverted in the miniseries adaptation of Gormenghast. Steerpike claims everything he has done was for love of Fuchsia, but while he may or may not have felt something for her, this declaration is clearly a lie by which he hopes to further manipulate her.
In Noah's Arc, Guy's love for Trey leads him to do progressively crazier things, including setting up a fire for him to put out to look like a hero, and cutting himself and blaming it on Alex. The cross into evil though comes when Guy writes a fake suicide note from Alex, heavily implying that Guy planned to kill him to get him out of the way for good.
This was Alpha's motivation for attacking Whiskey after his composite event in Dollhouse. He just wanted Echo to be the best, after all.
Really, this is a large part of Alpha's schtick—his break-in near the end of season one was to get Echo out. When he reappears in season two, it's killing all her clients who loved her—or as he put it, loved part of her.
In Jekyll, a modern retelling of Dr Jekyll And Mr Hyde, it's revealed that the psychopathic Hyde is not a manifestation of all the dark impulses of human nature as commonly believed, but represents the ruthlessness of pure love, prepared to sacrifice anything and anyone for the object of his affection. The Split Personality was created in the first place when the original Dr Jekyll fell in love with his servant who due to the class difference he couldn't marry.
Over a century later, Tom Jackman earns hisSplit Personality after he and his wife are menaced by a gang of thugs; Tom's frustration over not being brave enough to protect his wife leads the newborn Hyde directly to the gang's leader. Cue Extreme Melee Revenge.
This happened in Charmed a few times. Leo willingly became an Avatar to save his wife and before that, he killed an elder to save his son.
Phoebe willingly became the Queen of the Underworld to stay with Cole.
And Cole had absorbed all sorts of demonic powers to be reunited with Phoebe. And he's the only one they didn't forgive.
EVERY Soap Opera that has turned a beautiful, intelligent woman with numerous other appealing qualities into a psycho bitch hell-bent on doing everything she can, up to and including murder to hang on to a man who doesn't love her, despite the fact that she could easily get another.
Chris Keller in Oz kills every guy that Beecher sleeps with, including the guy who murdered Beecher's father, has Beecher murder Schillinger unknowingly, acts overly aggressive with him at times, on two occasions has knocked him out with a blow to the head when Beecher goes against his wishes and later handcuffs him to a chair away from other people where he forces a kiss on him, and finally murders the Aryans so that they don't pose a threat to him or Beecher anymore.
As well as Ryan over Dr. Nathan. After she helped him through cancer, he immediately divorces his wife. Later, he forced a kiss on her, paid a fellow inmate to steal her stethoscope, and when she started ignoring him, had his brother Cyril kill her husband. After she was raped, he very quickly murdered the man that did it. It was even said outright that his love for her was more of an unhealthy obsession. However, Dr. Nathan actually falls for him in the end.
Stargate SG-1: The Replicator Fifth's obsession with Samantha Carter heightens his reaction to her betrayal, turning him from a nice guy who wants to help SG-1 to a dangerous foe who kidnaps and tortures Sam, tries to trick her into staying with him, and finally creates a Replicator version of Carter to rule the galaxy with him. It doesn't go quite as planned.
First, we have Anastasia, whose entire motivation throughout the series has been to correct the mistake she made in abandoning the one she loves. She's done some horrible things. Though evidently nothing bad enough that she isn't fully forgiven by series end and reconciled with Alice and Cyrus.
Second, we have Jafar, who craves his father's affection more than anything, and will stop at nothing to get it.
Finally, we have Amara, who, after losing her children, became a sorceress with no qualms about killing an innocent man to use his liver as a spell reagent, and turning another man to stone for shiggles.
Every other episode of Warehouse13 has someone abusing artifacts out of desperation caused by love. The major difference between these episodes and ones where the bad guys are truly bad guys is the ones based around love generally don't leave as much of a body count.
Walter White of Breaking Bad actually has this as the endearing motive for his actions. His entire journey into meth manufacturing begins because he loves his family and doesn't want to die leaving them with nothing but a mountain of debt from his medical treatments. The show isn't even entirely through the first season, however, before it becomes clear this isn't the whole story and Walter is also driven by much darker motives he doesn't want to recognize.
Uther Pendragon from Merlin started his genocide of every single magic user when one killed his wife Igraine. Even though he had no one to blame but himself.
Tony Almeida learns the identity of the man whose actions lead to his family's death and attempts to get close to him so he can kill him in revenge. He does this by murdering the head of the FBI, performing a number of terrorist attacks, and attempting to expose a crowded subway terminal to a lethal pathogen.
Jack attempts to assassinate the mastermind behind the death of his love interest in the final season even though doing so would kickstart World War III and recklessly endangers innocent lives in his attacks on the conspirators.
And then subverted, as he is talked down when his allies make him realize what the consequences will be; and because they point out his lover wouldn't want this.
Ryuki's Shiro Kanzaki started the Rider War and is just fine with the Mirror Monsters devouring people if he can save his sister. Noble goal, but we're talking a body count in the hundreds.
In Wizard, we have the White Wizard and Wiseman, the Big Good and the Big Bad, respectively, who are both Sou Fueki. Apparently, saving your daughter outranks the fact that everyone with magical potential will become a Phantom and everyone without it will drop dead if your spell to steal the mana from everyone in Tokyo goes as intended.
In Gaim, Mitsuzane/Kamen Rider Ryugen starts off as The Lancer, but his obsession with protecting his friends (especially Mai, who he has feelings for), leads him to join the villainous Yggdrasil Corporation and become both a Manipulative Bastard and Poisonous Friend.
Sound Horizon's Elysion Paradise Illusion Story Suite album is full of seemingly nice girls doing really terrible things due to love (like the girl who loves her brother so much that she stabs him to death so they can go to Paradise together, or the one who kills the girl she loves for rejecting her...)
The Mountain Goats song 'Love Love Love' is sorta about this. It's off an album about the songwriter's stepfather abusing him as a child; his point was that his stepfather may have done terrible things to his kids, but he still loved them, and love can make you do terrible things.
Love love is gonna lead you by the hand/Into a white and soundless place/Now we see things as in a mirror dimly/Then we shall see each other face to face
Dschinghis Khan's "Der Verräter" (German for "The Traitor") is about a guy, who went AWOL from gate guard duty to see his girlfriend and allowed the enemy to enter the city. Afterwards, he's brought up on charges of treason and sentenced to death by hanging and everybody believes, that him claiming to have been seduced is only a pretext for the collaboration with the enemy and that his real motivation was promises of wealth and honor.
Downplayed in Mark Morrison's "Return of the Mack". The song is about a Casanova who fell in love with a woman who promised to never leave him. She did, and left him heartbroken, prompting him to return to his womanizing ways. So it's less "love makes you evil" and more "love makes you kind of a dick".
Giuseppe Verdi loved the trope, as proved by:
Count di Luna in Il trovatore.
Amneris in Aida.
Eboli in Don Carlo.
Abigail in Nabucco.
Gérard in Giordano's Andrea Chénier.
Scarpia from Puccini's Tosca is a subversion - it's not love, it's lust, and he was pretty evil to begin with.
Claggart in Britten's Billy Budd, if you interpret it as Foe Yay.
Alberich from Wagner's Tetralogy "Ring of the Nibelungs". Rejected by the Rhein Nymph Sisters for being a Gonk, he curses love and steals their gold, creating the infamous ring.
Jimmy Jacobs found love in Ring of Honor, unfortunately it was with one half of the Minnesota Home Wrecking Crew and former Special K member Lacey. The only thing not predictable about the results was just how bad they were.
In the Fellowship of the White Star event "London Fires," the arsons are caused by a woman who is in love with a heroic NPC who once saved her from a fire demon. She is setting various locations in London on fire in an attempt to meet him again and properly declare her love for him.note This seems to be based off old Japanese Folklore.
This is yet another trope that the Ravenloft setting was (literally!) built out of. From Strahd von Zarovich's obsession with stealing his brother's fiancee, to Victor Mordenheim's determination to restore his wife's shattered body at any cost, love — or, at least, what villains claim is love — is a far better candidate for the Root Of All Evil than money, in the Land of Mists.
In a sourcebook for the D20 Star Wars role playing game this is used as a justification for Jedi not being allowed to love. More often than not it leads to anger, jealousy and the dark side.
Warhammer and Warhammer 40,000: the Chaos god of decay and disease Nurgle is the embodiment of the emotion born out of man's reaction to death, usually interpreted as the emotion of love. And indeed, of all the Chaos gods, he's the nicest to his cultists (who refer to him as Papa Nurgle or Grandfather Nurgle), showering his faithful with gifts. Followers of Nurgle then seek only to bring the joy of pestilence and death to the universe.
Nessarose, Elphaba's beautiful but unstable sister from Wicked. Her obsessive love for Boq leads her to enslave the Munchkins so he can't leave her. When she discovers he doesn't return her feelings and has really loved Glinda all along, she shrinks his heart so he can feel her pain; Elphaba has to turn him into the Tin Man in order to save him.
Later, in the song "No Good Deed," Elphaba's transformation into the Wicked Witch of the original Wizard of Oz is significantly assisted when Fiyero throws himself to an angry mob to save her. She frantically tries to magically protect him from afar, utterly unsure that she is doing him any good. In frustration, she declares that "no good deed will I attempt to do again," and is a Nietzsche Wannabe for most of the rest of the show.
One of the oldest deconstructions of this trope occurs in William Shakespeare's Richard III. Richard, claiming that because he is "deformed" no woman has ever loved him; as he sees it, there are but two courses in life for men to take - that of the lover, or that of the villain - and because no woman would ever allow him to be their lover, he must therefore become a villain. This, therefore, turns the trope on its ear, into Lovelessness Makes You Evil, leading to a very Cry for the Devil idealism. (Shakespeare then subverts Richard's excuse by showing him, in the very next scene, successfully wooing the widow of a man he killed.)
Judge Turpin could also qualify, though much like Frollo above, there's more than a bit of lust involved as well.
Older Than Feudalism: Medea murdered her brother and betrayed her people for her love of Jason. Then in the play he leaves her, and she goes a bit murderous then as well, though that's more about revenge.
Medea: Oh, what an evil power love has in people's lives!
The framing device for Thrill Me is Nathan's parole hearing, where he explains his crimes (theft, arson, murder) as being due to his feelings for Richard.
But you've asked a simple question And I've told you why It wasn't on a dare or on a whim It's hard to comprehend now That the reason why Was simply that I went along with him
In Dragon Age II, someone is kidnapping women in Kirkwall because they resemble, in some way, the kidnapper's lost love, and is using their bodies to reconstruct her. One of the victims is Hawke's mother, who shares a face with the lost love.
Sophia Lamb from BioShock 2 - Not only does she say multiple times that she's trying to kill you and destroy free will because she loves all sentient life, she is willing to turn her own daughter into a soulless, brainwashed monster. Because she believes free will is evil, and by removing her daughter's free will she is "saving" her....rrrrrrrrriiiiggggghhhhtttt...
And, it's possible for you, the player, to become this. Your main goal in the game is to get to your "daughter" Eleanor. If, along the way, you choose to make the evil choices, it's all to save her.
Aribeth, the initial quest-dispensing NPC of Neverwinter Nights, does a Face-Heel Turn and leads the villain's army against the city of Neverwinter in vengeance for the unjustified execution of her fiancee at the end of the first chapter
The main villain of Tales of Heartsaccidentally engineered a parasite that drained his entire planet of life and was stopped by the sacrifice of one of his closest friends, and wants to re-awaken that parasite thinking that he can bring her (and his planet) back. Actually, it'll just eat more planets, but he's too crazy to notice.
In Super Paper Mario, Big Bad Count Bleck was once Blumiere, a young noble of the "Tribe of Darkness" whose romance with a human girl, Timpani, was ended abruptly by his father, who banished Timpani to the farthest reaches of the multiverse. So enraged and despondent was Blumiere that he turned into an Omnicidal Maniac, devoted to bringing about the end of all worlds by carrying out the prophecy in the Dark Prognosticus.
First, according to Lament of Innocence, he first became a vampire to become immortal and curse God's name forever in response to his wife's death.
Leon calls out Mathias on this in Lament of Innocence, telling him that Mathias went against what his lover would've wanted him to do (he basically said "did you lover ask you to do this? The Mathias I know would never have loved such a woman!") Leon was understandably pissed off since Mathias' schemes led to Leon's girlfriend being killed by Walter, but was satisfied by simply fulfilling Sara's last wishes.
Centuries later, the death of his second true love in a witch hunt convinced him to seek humanity's extinction.
This would prove Leon's point again. Lisa explicitly asked for him not to hate humans. Granted, he didn't know her last words till it was too late (but, considering she's a healer who tried to help her village...), but her death is when he starts hating and killing humans. So, basically, Leon's speech could also be a call-forward to Symphony of the Night, and also a glorious note of how History Repeats itself.
In the Sorrow games, Soma Cruz can either resist or succumb to Dracula's influence, depending on whether or not he believes that Mina is killed. It's a trick purposefully meant to turn him into Dracula by Ceila, but if it works, it backfires on her.
There's also Brauner from Castlevania: Portrait of Ruin, who became a vampire who sought to destroy humanity because his daughters were killed during the war.
It's implied in Clock Tower 3 that his incestuous obsession for his daughter (and later his granddaughter) was what originally turned Lord Burroughs into the Big Bad of the game.
Both played straight and mocked in Metal Gear Acid 2, where the token Evilutionary Biologist turns out to be attempting to reproduce his dead wife's brain in a little girl, bringing her 'back to life' - and is convinced that doing so would turn her into an all-three-faces-of-Eve-at-once God figure (and, just for kicks, start a nuclear war to clear out room for his kingdom). It's mocked when Third-Person Seductress Venus says it's so romantic that someone would do all that for love, after which Snake points out that it's probably the opposite of romance, and quietly marks her down as a Yandere.
And that's just in the first game. In the sequel, he goes back in time, captures the soul of the first game's protagonist's sister, uses it to capture the soul of the king of the gods, and steals a Cosmic Keystone to create a new world, all to get the attention of the goddess he's obsessed with. Although this time he doesn't get away with it.
You could make a compelling argument that this happens to Lenneth, despite it being portrayed positively and causing the best ending.
If Lenneth doesn't pursue her love of Lucian, then everything goes incredibly well and you win Ragnarok with no casualties.
If she does pursue her love Loki kills most of the gods and destroys all of creation. It Got Better though, which is the only redeemable factor. Even so, despite being less spectacular the end result from the B-Ending is better than the love story A-Ending.
In The World Ends with You, Beat becomes a Reaper at the end of the first week, believing that he can become the next Composer and bring his erased sister Rhyme back.
His evil is limited to attacking Neku a few times, as he never erases any other players. His disgust over what the Reapers want him to do leads to a Heel-Face Turn.
Used by Raphael Sorel in the later Soul Calibur games, where he essentially tries to infect the world (with his plague that causes suspiciously vampiric traits) for a little girl, his adoptive daughter Amy. How far this love goes is up to you.
In the second episode of Trauma Center, Patrick Mercer's whole reason for researching Neo-GUILT is to find a strain that allows for quick regeneration and bring back his comatose wife Tracy. When Bythos doesn't work as expected and puts Tracy's life in danger, Mercer refuses to give up and tries another strain, the dreaded Aletheia, by first experimenting it on Ms. Mayuzumi. Once again, this fails horribly, and Mercer ends up getting killed by U.N. soldiers when he tries to bring everyone down with him.
Partially played straight in Final Fantasy IV, as no-one's entirely sure how much of Kain's betrayal is due to his unrequited love for Rosa and how much is due to Golbez's mind control. He later admits that he's not entirely sure either.
Shadow of the Colossus plays it fairly straight, depending on where in the Alternate Character Interpretation you fall. Wander's whole motivation for coming to the Forbidden Land, making a deal with a dubious sealed entity, and slaughtering peaceful giant creatures is to revive his dead love.
Considering the manual states Wander traveled for 'many moons' to reach the Forbidden Land, it's an awful long way to go, suffering the discomfort of horse riding for extensive periods, for someone one hates. Regardless of whether it's due to being a relation, friend or lover, it's simply a lot easier for people to think that Wander went through all that for love.
Mystery of the Emblem's Hardin fell into despair after finding out that his longtime Bodyguard Crush and newly-married wife Nyna didn't truly love him back, leaving him open to be corrupted by the Darksphere.
Genealogy of the Holy War's Arvis loved Deirdre so much he brainwashed her into loving him and killed her husband Sigurd out of fear that she might remember him.
Blazing Sword's Nergal originally wanted to bring back his dead wife, hence his obsession with harnessing the power of the dragons and delving into dark magic. It eventually stole his soul and transformed him into the power-mad Big Bad of the game. Averted with Dorcas; he joins a gang of bandits to earn money for medicine for his Ill Girl wife, but you can have Lyn talk some sense into him, tell him that the wife is her protegé now and convince him to join her side.
Fire Emblem: The Sacred Stones has Orson (he betrayed his country to have a chance to revive his wife Monica, ex Ill Girl who died prior to the game events., General Carlyle (Queen Ismaire's long-time Stalker with a Crush) and Prince Lyon (he adored both Eirika and Ephraim and was at the same time jealous of them since he was an Ill Boy, which contributed to his madness later).
Fire Emblem: Path of Radiance has Ena, who was fighting for Daein in an attempt to save her boyfriend Rajaion, who was captured by Ashnard and then turned into his mount. This also causes her grandfather Nasir to betray Crimea.
Jolee Bindo of Knights of the Old Republic, four thousand years before the events of Star Wars, knows that the Jedi of his time are shifting to the view that Love Makes You Evil, and having lived alone in the forest for twenty years thinking back, makes the case to the player that this isn't so.
"Love doesn't lead to the dark side. Passion can lead to rage and fear, and can be controlled, but passion is not the same thing as love. Controlling your passions while being in love, that's what they should teach you to beware, but love itself will save, not condemn you."
Bindo's observation comes from some hard experience. He was too head over heels with his wife to see fault in her, and trained her as a Force-user against orders. She eventually ended up joining Exar Kun's army. He also mentions knowing Nomi Sunrider, meaning he probably also heard about what went down with Ullic.
Also, part of the reason Bastila turned to the dark side (at least if the player is male) was that she was terrified of falling in love with him.
I've been meditating on something. If we marry and start a family—yes, I know things are hectic right now, and there's a lot of decisions to be made, but if we ever do—I wouldn't want to keep it a secret. Almost everything I've studied about the "perils of love" seems to be written by Jedi who've never experienced it themselves. They talk about "uncontrolled passion" and the "fear of loss" like they're poison. But these Jedi never discuss devotion, patience, and compassion. Love's taught me more about those than the rest of my training combined. Those Jedi just dismiss love as "obsession," when love's the opposite and they don't seem to know it. If the Masters are going to claim that emotional attachment is flat-out wrong, I want to know that they've experienced it, and still think that way. Love can make a Jedi less selfish, more humble and more devoted. We're the proof. I wouldn't want to hide that from anyone.—Nadia
A quest chain in World of Warcraft has you track down old journals and records and undelivered letters to piece together how one Stalvan Mistmantle fell for a girl he'd been hired to tutor and murdered her, along with her whole family, when she didn't requite him. In the new expansion, one of the wishing coins you can fish out of the Dalaran fountain seems to indicate he first looked for a Love Potion:
"You mages refuse to provide me that which hastens the inevitable fate of two people in love, when only one is too naive to see it. I wish you all ruin."
The root of Fandral Staghelm's descent into evil is the death of his son in the War of the Shifting Sands. Xavius thus manipulated him into grafting a corrupted twig onto Teldrassil by using an image of his son.
Tragically repeated in the latest patch. One of the more prominent flame druids, Leyara' backstory reveals that she was in love with Fandral's son, as well as the bearer of their child, and the dual losses of both to tragic circumstances made her insanely vulnerable to following Fandral right down into the depths of Ragnaros-based corruption
Alma from F.E.A.R. is pretty much like this. Then again, if you don't wish to be loved by her, she'll just rape you.
The King of the country of Valentine had a daughter who he treasured above all else. So much that he felt betrayed beyond all reason when she fell in love with the king of the enemy kingdom and became pregnant with twins. He made life a hell for his grandchildren, and eventually strangled his own daughter, whom he said he treasured so much he wouldn't let rain fall upon her, with his bare hands. He was killed in a magical explosion in a war, but when he reappeared years later, having bargained his way out of the Netherworld, he effortlessly reduced his confident, self-assured granddaughter to being both paralyzed and trembling with open fear—just by talking to her. He also hatches Leventhan, the Dragon which will help bring about Armageddon and does many other terrible deeds, while, at some level, hating himself for all that he's done. His self-loathing is to the point that in a candid moment, King Valentine begs his daughter's lover to crush his undead form and stop him now.
Another love-related emotional scar happens to the Black Knight, Oswald. Oswald is a human raised by a Fairy noble, Melvin, who he loves as a father and would do anything for without hesitation. Melvin doesn't return the feeling, and, as he's dying at the end of a failed coup, freely admits that Oswald was only an object, a tool to help Melvin gain the throne of the fairy nation. Ever after, Oswald reacts strongly—sometimes with lethal violence—whenever he thinks someone is thinking of another person, or even herself, as a mere thing.
Magus of Chrono Trigger - he in fact doesn't give a rat's ass about world domination as is implied in the early game, and is instead single-mindedly pursuing revenge on Lavos over his sister, with no regard for the well-being of anyone else in the world.
Shadow the Hedgehog in Sonic Adventure 2. After he loses Maria, he promises revenge, and mistakes Maria's last wish for happiness, as the other way around. He tries to destroy the world, and it's only after a talk with Amy, he relises his mistake, and corrects his mistake.
Shadow in his own game can go either way. If gone down the dark side, Black Doom revisits Shadow's memories of the attack on the colony, and can than change his ways, or go insane with power and destroy the world!
Viking: Battle for Asgard: Rakan fell in love with Freya and was subsequently rejected by her. In her defense, the narrator explains that mortals are rather fragile creatures and a relationship with a God would probably kill them.
In Professor Layton and the Unwound Future, it's revealed that the reason Don Paolo hates Layton so much is over a woman at their college, though Layton was rather oblivious to the his feelings about the subject at the time. Specifically, Claire and Layton were a happy couple, and Paolo couldn't stand to see her with him. One of that game's antagonists was also driven by his feelings for the same woman, though his hatred is not so much at Layton as the one responsible for her death.
One of the main themes of Rule of Rose could be said to be "love screws you up, and makes you treat others horribly". Most notably Gregory going crazy over the death of his son Joshua, and becoming the serial kidnapper and killer known as the Stray Dog, as well as Wendy whose innocent children's love with Jennifer is the sweetest thing ever, up until she thinks that Jennifer loves her dog more than her, and has him killed, and that's just the start.
In Cursery: The Crooked Man, Count Blaise Morellus was so in love with his fiancee that he tried to lock her in their chateau at all times. When she escapes one night, his attempt to get her back leads to her death from falling off a cliff. Driven insane by grief, he accepts a cursed gift from Mother Goose and becomes the Crooked Man, kidnapping and killing young women in hopes of finding his reborn fiancee.
In European Mystery: Scent of Desire alchemist Gaston's wife was accidentally disfigured when there was an explosion in his lab, so he started kidnapping and killing young women so that he could use their essence in a potion to restore her beauty. She rejected him when she found out what he did, which drove him even more insane.
In the Jumi arc, Alexandra turns on her own race to save Florina, bringing them to near-extinction in the process.
In the Dragon arc, Larc made a Deal with the Devil to gain more power to be able to protect his sister, Sierra.
In the Gate To Heaven arc, the whole plot is kicked by a Love Dodecahedron between Irwin, Escad and Matilda.
There are other smaller examples that border between this and Love Makes You Crazy... or, at the very least, Love Makes You a Jerkass. Somewhat justified in-universe, as it's stated that Mana Goddess is love, and she was corrupted and turned evil.
Love is one of the central themes in Mask of the Betrayer, the expansion/sequel to Neverwinter Nights 2. Putting aside more exotic variants like the love between man and god, the entire plot and background of the game, including the horrible fate that befalls the protagonist right at the beginning, occurred because Akashi and the Founder were in love. Myrkul finds it amusing.
"Only love could be so cruel, I think."
Denny and Capek, the first two antagonists of Shikkoku No Sharnoth. The latter is necromantic, and the former has some twisted motivation about becoming king in order to win love.
Hatoful Boyfriend has a route called "Bad Boys Love". Why is it called that? Most likely because it's about the human protagonist being Killed Off for Real, her main love interest being traumatized to hell and back, and the genocide of the entire human race being set up all because of Doctor Shuu's unrequited love for Ryuuji Kawara that caused him to do his best to fulfill his promise to said love to fulfill any wish of his sonin the best way he knew how. Of course, Doctor Shuu wasn't the most stable of creatures to begin with, but it doesn't change the fact that none of this would have happened if he hadn't loved Ryuuji enough to take his promise to him so horribly seriously.
Angie Gallant: All he has done, he has done for love. HORRIBLE HORRIBLE EVIL UNEARTHLY INFERNAL LOVE.
Roa of Tsukihime was apparently a very pure and probably decent person before apparently falling in love with Arcuied at first sight. He decided it was hate for some reason, so he tricked her into drinking his blood, making him a Dead Apostle and making her kill off all the other True Ancestors. Apparently all he wanted was for him to like her...
To expand on that, Roa was a completely pure priest and member of the Church's Burial Squad. He was doing some research on immortality until the point where he first saw Arcuied. The moment he saw her, he fell in love, but because until then he had never felt either love or hatred, he was confused. Thus, he decided that the pain in his heart must be hate; It's actually quite a tragic chain of events.
For the first time since he was born. Probably, no definitely, the only time in his life. -Michael Roa Valdamjong fell in love with that white woman- "————" This is the first time. Not an impulse, but truly Roa's heart. The only remaining emotion. Roa's personality had long died, but it continues to exist without disappearing, that eternal memory. "?? I, see." That's why—-he hated Arcueid so much. The woman who stole his purity. Just an instant. He only saw her for an instant, but his heart was stolen. That hated True Ancestor that caused his purity to fall. That existence. He hated everything about that white vampire princess. "?? What a mistake." What a mistake it was. Roa hated her so much that he reincarnated so many times and always waited for Arcueid to come pursue him each time. For that, he did anything. He deceived Arcuied, fooling her when she didn't even know she was a vampire, and let her suck his blood. Becoming a dead apostle of Arcuied, he used her power to destroy the remaining True Ancestors, and waited for her. Why did he not understand? The hate that caused him to reincarnate and wait for Arcueid. That isn't hatred. The man called Roa was so pure, he didn't even understand his own emotions. To think about someone else so much it makes you go almost crazy. That feeling is very similar to hatred. But, merely a single word. If this man called Roa could have been told his feeling was love, he wouldn't have made this mistake—-
Caster from Fate/stay night. At one point in UBW her arrogant, calculating mask slips and she's absolutely scared to death about her prospects. She's started doing horrible things, much worse than she did as Medea in pursuit of a goal she doesn't believe is actually possible. It's not helped along by Kuzuki's incredibly emotionless behavior.
Higurashi: When They Cry gives us a case of psychosis and mass murder over a bad case of one-sided Love at First Sight. And psychosis and murder over brotherly love. Twice. Essentially, Love Makes You Evil andcrazy. This is played straight with Shion, but subverted with Tomitake, who is horrified at Takano's plans.
In Sluggy Freelance, Oasis's love of Torg has taken her into serious Yandere territory, complete with trying to kill anyone she sees as a rival for Torg's affection(including chocolate ice cream), chaining Torg up so she can force him to marry her, and at one point planning to sow herself to Torg with industrial strength cable so they can never be apart. As seen in the "Phoenix Rising" arc, when she's not actively pursuing Torg she's ... well, not exactly good, but definitely more of an Anti-Hero than a Villain.
However, in Oasis' case, her insanity is due to her strange and large unrevealed abilities, which allows her to be resurrected from death, but radically alters her personality Solomon Grundy style; she could be completely Ax-Crazy one incarnation, and rational and contemplative the next. On the other hand, before meeting the heroes she was mind controlled by a villain, whose brief order to love Torg has been thought to have conflicted with later orders, which is thought to be the root of her insanity in the first place...
Even though Oasis herself came up with the idea, it's not at all clear she actually changes personalities between incarnations, as opposed to just as a reaction to different events and circumstances as part of her instability.
There was also the "Love Potion" arc, where the titular Love Potion brings out the most violent and destructive side of love in everyone it's used on.
Except Bun-bun: "These lightweights just can't handle their evil."
It should also be noted that K'Z'K was deliberately twisting the spell to be more evil.
The Order of the Stick gives us Vaarsuvius'ssoul-splice arc. Although lust for power and control had a big influence on the eventual decision to take the fiend's deal, the initial motivation for contacting them truly was out of fear for Inkyrius and the children.
Growp: You're like a work of art, Emily. Battling you fills me with emotion. I've never met someone who could match my ferocity, my brutality. Emily: If you adore me so much, stop trying to kill me. Growp: You don't understand, do you? To kill you would complete this piece of art, would be an expression of life, of being. And your death can only be art if the kill is made by the master skill of a hand as poetic as mine.....Grrr! What I'm saying isn't coming out right, what I'm trying to tell you is... Is that... I... I love you.
In Haru-Sari, we have Chi-Min, who could've healed June's Calphanika's, but because he wanted her to need him. Alberich may also count, he stated that'd he'd do anything for Phoenix. ANYTHING.
Lana from MSF High, to the point she beats up anyone who makes eye contact with Donovan.
Aaronev Wilhelm Sturmvoraus from Girl Genius. Considering his family's history, he probably wasn't the best or the most sane person around before he met Lucrezia Mongfish, but it was his obsessive love for her that made him even more crazy and ruthless, to the point he sacrificed possibly hundreds of young girls (including his own daughter) to bring her back.
Sailor Nothing: the Dark Queen unintentionally created the Yami-gaia in an attempt to save her lover from dying of consumption.
Zig-Zagged in Tasakeru. In the Mythology, two of the settings death-gods come about from mortals corrupted by unrequited love (The Scarred One, who had the good part of his soul die upon being rejected by the object of his affections and subsequently being disfigured by her lover, and Aconite, who was so desperate for the perfect husband she offered to submit to him utterly and became the embodiment of decay when her sisters slapped her with The Punishment), while in-story, love triggers two of the main characters ascention to demigods.
In Worm, Trickster betrays his teammates for Noelle, his ex-girlfriend who had gone insane and become an Omnicidal Maniac.
Berry from Foster's Home for Imaginary Friends entered the house sweet and innocent, but fell in love with Bloo and became a psycho, a condition which appears to only get worse the longer she's in love with him. The second time we see her she is genuinely homicidal. This may have been her true personality all along, however.
The ultimate irony is that in an alternate timeline, Vlad ends up marrying Maddie, but it's not a happy one due to the fact that he's a total control freak. What's more, Maddie still prefers Jack.
Harley Quinn, although listed above, did originate in the DCAU. She is shown to be a generally good (If crazy) person when on her own. Being in love with The Joker though, brings her to all the depths one would expect.
Other characters in BTAS that represent this trope are Mr. Freeze and The Mad Hatter.
Nox, the Big Bad from Wakfu. His love for his family that he lost 200 years prior to the show drives him to do terrible things. Up to and including genocide.
Oroku Saki/The Shredder in Teenage Mutant Ninja Turtles (2012) was in love with the same woman Hamato Yoshi/Splinter loved. When she chose Yoshi over Saki, he became jealous of Yoshi and came to hate him. His entire plan is to kill Splinter by using his long-lost daughter as a weapon.
One of the reasons why Trina Riffin from Grojband changed her name from Katrina, is to win over Nick Mallory.
The infamous Long Island Lolita case of Amy Fisher. A sixteen year old girl who shot Joey Buttafuoco's wife in the face. Fortunately, she survived. Joey Buttafuoco was having an affair with the teenage girl, despite being twice her age. The case got national attention during the early 90s and many made-for-television movies were created based on the case. These films were famous for starring known eye-candy as Amy Fisher. For example: Alyssa Milano and Drew Barrymore.