Lilith is a rare sympathetic example. She was normal and sane, but was mentally tortured by her absolute horrifying monster of a "husband" Desus so badly that she shattered into a mental, slightly Ax-Crazy wreck who was a sad little puppet for Desus. By the period the game takes place in, she's recovered - somewhat, and this is after going feral and living in pure chaos for the better part of 2,000 years - although she tends to understandably explode into a frothing rage whenever she sees something related to Desus - which is unfortunate for Desus' reincarnation, Swan, who, while not a hero exactly, is still a lot better person than Desus ever was.
Adorjan is a Yozi who's gone seriously Yandere. One of the more powerful Charms she offers to her chosen does ridiculous amounts of damage, but has the side effects of a) doing some of that damage to yourself, and b) getting Adorjan "interested" in you if you ever use it on her. As the Quotes page says, her hatred is safer.
Also from the Yozi pile is Kimbery, who's a cross between this and every overbearing mother ever. She loves her "children" with all her heart and ladles them with all sorts of gifts... at least, until she thinks that they'll betray her, at which points she drowns them in poison and acid, both literally and metaphorically.
The Galateids from Promethean: The Created tend to have this as their Torment. Seeing as they embody the sanguine humor, their periods of Torment tend to involve giving into murderous obsession over others; the book lists the example of abducting an unrequited lover's girlfriend, holding her at knifepoint, and demanding the lover choose between the two.
Legend of the Five Rings allows you to give your character a yandere stalker via the Bad Fortune: Secret Love disadvantage, netting you a few extra Experience Points at the cost of having some unknown Non-Player Character out there in the world who will stop at nothing to have your character all to him- or herself.
The Deceivers from the Third Edition of Nobilislove everything and think that the best way to express that love is to strip things of the toxic lies that enshroud them and hide their true nature. Lies most people call "existence."
In Warhammer 40,000, the Chaos god Nurgle fell in love with Isha, the Eldar goddess of life. After Nurgle rescued Isha from Slaanesh, he expressed his love for her by imprisoning her in a cage and killing any warriors who attempted to rescue her.