In fact, I'm a "thought-yandere" myself, which means by thought, I have reached the level of Yandere to occur, but is bounded by some important thought from commiting an act. I am bounded by religion and law and have not released my inner self. I would have done fucking crazy batshit if it wasn't for those rules. To prove it, my farthest whim of yandere-ism is to try to kill the boyfriend of the girl I like, peel his facial skin off, and wear it on my mask and became good friends with the girlfriend I like.
Yanderes majorly has depression, psychosis, or some other mental illnesses, that induces the victim to lose contact with reality and lose the basic ability to differ fantasy and reality or good and evil. It majorly happens between teens as their minds had been alterred by a period called "teen angst". However, the whims that drive the yanderes to action are majorly the evil instinct of people, the love only takes about 10%. Yanderes share many characteristics with the criminals in school shootings, as they are teens, and they kill. Its only their reason they did it that they differ.
TL;DR: You know what's a yandere? It's the guys who commits the school shootings.