"But don't worry! I'm not going to kill you - oh, no! I mean, I was going to, but then I had a really good idea! See, I'm thinking I'll just core your stupid brain and make you my zombie slave - hee hee! You're going to be so in love with me... you will soooo regret choosing her over me!"''
Oasis, as a result of being Brainwashed and Crazy, is generally "giddy as a schoolgirl", but will kill anyone or anything that stands between herself and her "love" Torg.
During Chapter 18's The Love Potion, Gwynn accidentally whips up a Love Potion which is a little too strong. Dex, Crystal, Riff and Bun-bun all become Yandere for the duration of the spell.
Monica, Riff's last girlfriend, is manipulative to the point of emotional extortion, and wouldn't not take breaking up for an answer. Though her violence never became murderous, she was revealed to be a possessed agent for a demonic cult that placed very little value on human life.
Curio from Templar Arizona, under her cute and bubbly exterior, turns crazy and weird not even over a love interest, but over friendship interests.
Zimmy from Gunnerkrigg Court does this to her only real friend Gamma, to the point she "translates" other people's words (Gamma doesn't speak English) to make Gamma think everyone else hates her. Nicely embodied in this quote:
Zimmy: I love her! I'd kill everyone in the world and then myself if she wanted it!
Kat: That's pretty sweet. Except for the part where you kill everyone.
Snaug has shown definite Yandere tendencies toward Moloch, who seems oblivious to the subtle threat in her affections. Notable examples include This and This. Whether or not he ever catches on (or she catches him) remains to be seen.
Tweedle claims no romantic interest, rather ones involving politics and allies, but he's so intent on playing out his personal script with Agatha as his 'princess', that he effectively enslaved Agatha by chemically changing her and himself so he has to touch her ever so often for her to remain healthy. When Gil shows up looking for Agatha, Tweedle almost goes off his rocker trying to kill 'Evil' Baron, Jr.
Orphaner Dualscar, Eridan's Famous Ancestor, He had redrom feelings for Mindfang despite their long-standing blackrom relationship. So when Mindfang started to show signs of redrom with a slave girl, Dualscar got jealous and had the slave girl killed. Then again, for Alternian society, this was probably considered normal. One could even make a case for Eridan himself being this - he reallydidn'thandlebeingrejectedbyFeferiwell...
Worse yet, the slavegirl Mindfang took was actually the Dolorosa, the Signless'/Sufferer's mother/lusus, and Mindfang wasn't actually red for her, she was using her to make Dualscar jealous in the first place. Her descendant Vriska was one for Tavros Nitram — alternating between redrom and blackrom for him, constantly attempting to be the most important person in his life, manipulating him constantly, paralyzing him via mindcontrol, keeping his sawed-off legs as a Creepy Souvenir, and finally killing him with his own lance through the heart.
Then there's that fact that Sober!Gamzee ends up cutting off Tavros' head, whom he had unrequited flushed feelings for, and keeping it around. His lips are covered in brown blood, and it's never specified whether he kissed Tavros before or after having decapitated him. Whether or not this counts as Yandere-ness is debatable, though, especially considering that Vriska was the one that killed Tavros in the first place.
Taken to its extreme by Damara, an explicit character of this archetype who follows a lot of other Japanese Media Tropes as well. Her romantic jealousy of Rufioh is so extreme that Meenah exploited it to make her snap, but apparently it caused her to have a Freak Out leading to her becoming an extremely horrible, crude person obsessed with random acts of revenge. (It should be noted, however, that she lacks the "sweet outside" portion of the trope, save for her brief conversations with Dave and Rose—around her fellow trolls, she makes no attempts to hide her psychotic demeanor.)
Sadoko in YU+ME: dream , as we learn late into the series. When she first appears, she's a somewhat suspicious, yet innocent appearing girl. Not soon after the Wham Episode her true nature is revealed.
Waya-Hime in Bravoman: Super Unequaled Hero of Excellence was hired to assassinate Bravoman but has a huge crush on him. Unfortunately for Bravoman she can't seem to comprehend that stabbing someone to death is not good for a relationship.