Kotonoha Katsura

Kotonoha Katsura is a major character in the mundanely named, but violently ended anime and visual novel School Days, and a classic example of the yandere character archetype. Kotonoha was a shy, but otherwise normal teenage girl until she started dating a boy named Makoto Itou. Over the course of the anime, Makoto cheats on her with several other girls, most notably a girl named Sekai Saionji, who quickly becomes Kotonoha's rival. In the anime ending Sekai murders Makoto for cheating on her, stabbing him to death with a kitchen knife. The scene is discovered by Kotonoha, who has at this time been driven to the brink of violence by Makoto's actions, severs Makoto's head and then sends a text message to Sekai, telling her to meet her on the roof of the school, where she then proceeds to cut Saionji's throat with a folding hacksaw. Kotonoha commits a similar murder in a "bad end" of the visual novel, slashing Sekai's throat on a pedestrian overpass, in front of Makoto and several others, then standing there laughing maniacally, suggesting at that point she was not aware of, or did not care about the consequences of her actions.