A visual novel turned manga/anime. The actual gameplay is the typical dating game. You play as Makoto Ito as you date and eventually develop a loving relationship between either Kotonoha Katsura, Sekai Saionji, or six other beautiful young women.more...
But it is at the endings when this game rears its ugly head. It is infamous for its three notoriously gruesome "Bad" endings, which either involve Makoto being stabbed by Sekai, Kotonoha committing suicide in front of Sekai and Makoto, scarring them for life, or Kotonoha outright killing Sekai while on a date with Makoto.
But when it came time to create an anime, Studio TNK decided that it wasn't horrifying enough. And thus, there was only one ending...the yandere ending, where Makoto is stabbed by Sekai, Kotonoha severs the head from the body, Sekai's neck is slit by Kotonoha, and Kotonoha lives Happily Ever After with what's left of her boyfriend/corpse.
It is worth knowing that there are actually 21 possible endings, including the downer endings.
If you watch the show, beware of the idiot plot and the yandere