Well to simplify, Girl A had best friend and boyfriend, we'll call best friend Girl B and boyfriend as just Boy.

Girl B had a crush on Boy and he knew about it while Girl A was oblivious to it (I think). They probably fooled around like Makoto did in School Days. Girl B wanted a serious relationship and got rejected. Next day, she comes to school and calls out Girl A during her class. Cat Fight outside, then Girl B suddenly pulls out a knife and stabs Girl A. WTF!? Yup, it happens, bag inspection was lax if you were a student and she apparently hid it on her bra. And the most shocking thing for people who weren't involved or didn't see it first hand, was that the guy wasn't anything special to fight over. Lol.

there's an american movie of a yandere girl. Not sure if anyone knows it. The guy is like an artist or something, and his neighbors have a daughter who's like 16 or under. And she's totally obsessed with him. I remember one seen he was sleeping naked with his girlfriend and she breaks into his house and pulls his blanket off to find them naked. And another seen he finds out she has a shrine of him in her closet. And i think it ends with her pretending he raped her and he gets sent to an asylum.

That's one of the problems with real "yanderes". As far as I can tell from news et cetera, they only obsess over the greatest assholes in existence. How can anyone be stupid enough to expect a real relationship from someone who already is cheating on his current partner? Naivity is moe, retardation isn't.

Lolz Isn't that the same for most manga/anime yanderekko.