Despair Event Horizon is when a anti-hero, anti-villain or tragic villain goes through a horrible experience that forces them down the path of evil and severs any hope for redemption in the villain's own eyes.
Examples[]
Cameron/BlackMetalGreymon[]
His lover Melody, the only person he ever loved (and who made him begin to turn good) was murdered on Kyle's rage. This tragedy turned Cameron into the heartless and brutal Digimon he is today, as it made him believe that he was destined to always be the monster and barbarian BlackGarurumon need in his team.
Damian Wayne[]
Displeased with the concept of being nothing more than a mere lab experiment, Damian Wayne destroys the laboratory. Shortly afterward, he meets Red Skull, who proposes a partnership with Wayne in exchange of helping him control his powers. Damian Wayne is "trained" over the next few months, being pitted against challengers in Hydra's gym and restraining them to capture. Damian Wayne eventually realizes that Red Skull is merely using him as a tool and destroys his headquarters. After defeating Red Skull, Damian Wayne flies back to the island where he was created and begins plotting revenge against humanity.
Jonathan Irons[]
After the death of his son, Will, Irons began plotting against the world's governments believeing the United States was to blame for his son's death and the belief that the world's governments actions to solve the world's problem only made matters worse.