Yandere/Fan Works

Admiral Daro'Xen, the Big Bad of the Mass Effect fanfic Project Gethinator, is crazy in love (or is that lust?) for Commander Shepard. She's gone into quite in-depth study of Shepard's history and psychology and has determined that Shepard is quite susceptible to the charms of female quarians, and nothing will dissuade her from the notion that she and Shepard were meant to be together. Everyone else working with her (particularly her second-in-command, Hvenna'Tor) considers this aspect of her to be utterly disgusting. It definitely doesn't help that she's written a collection of Self Insert Real Person Fics about herself and Shepard, with a reputation for depravity second only to that of Fornax's Forbidden Issue. When she and Shepard finally face off in the final chapter, when Shepard turns Xen down (because Xen is completely out of her mind and because Shepard is in love with Tali), Xen has an If I Can't Have You Villainous Breakdown of literally star-system-shattering proportions -- she turns the same mineral scanner that Tali's idiot crewman Prazza (who was responsible for the Forbidden Issue being labeled such) accidentally blew up a star with in the very first chapter of Inglorious Boshtets on Eta Carinae, causing it to go hyper-nova in a furious bid to destroy Shepard, Tali and the entire Normandy crew.
Office Politics: Raito towards L.
In the Death Note fic A Cure For Love Light begins a relationship with L during his Memory Gambit but when Light becomes Kira again he becomes very possessive and obsessive of L. He begins making up excuses to himself why he isn't going to kill him now like "it's not a good time," or "I want to break him first" (suggesting he really does love him underneath the psychosis. Light becomes very angry and upset when someone else tries to kill L.) Light arranges things so that Misa kills Watari, and then he kills her because he felt threatened that Watari was going to make L break-up with him and that Misa (a Yandere herself would try and kill L so that she and Light could be together). Light keeps L drugged up for weeks afterwards and spends so much time with him that his infatuation becomes rather obvious to everyone at which point Light even considers killing his own father because he's against their relationship. Later when L in effect breaks up with him, Light goes completely off the Morality Chain. He uses the deaths of his father and a former teammate to essentially takes over the world in an afternoon.
In Ragnarok (fanfic) Takada towards Light. When Light began dating her in college they agreed it would be nothing serious since they both wanted to focus on their careers. But when Light broke up with her Takada did not take it well and has held a grudge for four years and seems to be out to ruin Light's career.
47 Days to Change by Snow-owl01: An unfortunate tale of Harry's failed attempt to mold young Tom into a decent, law-biding citizen. Instead, as Fate will have it, young Tom grows up to become the same twisted psychopath, who is hell-bent on winning the love of his adoptive father. Harry's consent be damned.