Batman and the Joker have always had a Foe Yay context in their archnemesis relationship. Scott Snyder dragged it out in the open as a main plot point in Death of the Family, where Joker wants to get rid of both his and Batman's associates (Harley Quinn, the hyenas, Nightwing, Robin, et cetera) so that it can go back to being about just the two of them. Batman rejects the Joker, who does not take it well. This leads to Batman: Endgame, where the Joker decides that he's done playing around with Batman and wants to kill him, his sidekicks, associates, the Justice League, his villains, and all of Gotham City. And just to drive the point home that he's being serious, he reveals to Batman that he's known his secret identity all along, and even knew the location of the Batcave. Without Batman in his life, the Joker simply does not care anymore, and it leads to arguably his most monstrous portrayal yet.
The cover of #41 of Nate Grey's solo series pretty much spelled this out in terms of Maddie Pryor's attitude to him - she was depicted giant-sized, Hell-Bent for Leather, holding him between two fingers, and licking her lips suggestively, with the words "If Madelyne Pryor can't have Nate... No one can!" emblazoned across the cover. Oh, and remember, he's the alternate counterpart of her son. Which both of them are aware of, and only Nate seems to be bothered by. In general, Maddie tended to pop in and out of Nate's life on a whim, sometimes helping him, sometimes not, and getting insanely jealous whenever a woman showed interest in him, showing textbook Tsundere behaviour dialled Up to Eleven. She even wound up murdering her chief competition for Nate, Threnody (who didn't stay entirely dead, but that's another matter).
Oroku Nagi (The Shredder's brother) from the original Teenage Mutant Ninja Turtles comic books does this to Tang Shen, the mutual object of affection between himself and Hamato Yoshi. Nagi begins beating on Tang Shen until Yoshi arrives and, enraged, kills Nagi. In subsequent adaptations of the story, however, Tang Shen is killed by the spurned suitor, who is usually changed to be Oroku Saki (The Shredder himself).
This is the basic principle behind The League of Evil Exes in Scott Pilgrim
Monica's Gang: In one story, several mothers have been abducted and Monica and Maggy decided to investigate. They disguised themselves as a mother in order to find the villain's hideout. The villain explained about how he'd been bullied for not having a mother ever since she vanished so he decided that, if he couldn't have a mother, nobody would. His mother reappeared and spanked him for his crimes. Fittingly, the villain's name is Frédipo. (Fred + Édipo - Édipo being how Oedipus Rex is known in Brazil)
Eclipso feels this way towards Superman, after trying—and failing—several times to possess and/or corrupt him.
The light-hearted nature of Captain Marvel can distract you from Sivana being this in one story. Because Captain Marvel prevents his plan to take over the Rock of Eternity so he can take over the Universe, Sivana tries to activate a Proton Bomb to destroy the Rock of Eternity, which is the Keystone of the Universe and will cause its destruction, even killing himself, deciding if he can't rule the Universe he'll wreck it. He is even laughing at this as he wants one last laugh. Just look at his reaction—http://digitalcomicmuseum.com/preview/index.php?did=16428&page=32
The Venom symbiote feels this way towards Spider-Man ever since Peter rejected it. On one occasion Peter defeated Venom by pretending to want the symbiote back, and the symbiote immediately abandoned Eddie Brock.
In A-Force, Queen Loki is trying to vie for She-Hulk's spot as Baron of Arcadia, even going so far as to get America Chavez banished to the Shield to further her plans. However, when those plans are struck down and Nico rejects her We Can Rule Together request, she opts to just blow a hole in the Shield and let the Marvel Zombies take it instead.
This trope forms the climax of the EC ComicsTales From The Crypt story "The Thing From The Grave," where two friends in a love triangle has one of the men get jealous and decide that he should Murder the Hypotenuse to get the girl all to himself. She doesn't accept his advances and he snaps, claiming that all his effort in dispatching his rival was ruined, and that if he can't be with the girl, he'll just kill her as well since She Knows Too Much. Fortunately, she is saved when her screams for help literally wake the dead man by reminding him of his promise to protect her. She faints in terror from the fire set to kill her, but the man's zombified corpse feels no pain and carries her to safety before it turns on the murderous man and chases him to the forest where he'd interred his rival. The zombie decides that he's taking the murderer with him back to the grave just to ensure the girl is safe, and buries them both.
Green Lantern In the Sinestro's Law arc we have Sinestro repeatedly offering Hal to join his corps but promising that he will kill him if he continues to refuse.
Wonder Woman: Mona Menise decided to kill both Steve Trevor and Diana when she realized her one sided crush on Steve would never be reciprocated since Steve was already in love.
The Ultimates: When Ultron realized that the Scarlet Witch would only love her brother and would never have any feelings for him, Ultron shot her.