It's by no means uncommon in fiction for a good and benevolent person to turn to darkness and evil. Sometimes they're corrupted, they're brainwashed, they're misguided, there could be many reasons. This is the Face-Heel Turn
Then there is this trope. A good character turns to evil, but their turn is sudden and poorly handled. The character is shown to have little reason, if any, to change sides, and if they are given a reason it's a weak one. This can also come about if their character changes so drastically after the turn that you wonder how they were ever good. It's one thing for the edgy Anti-Hero
to fall to evil, but if a character has previously shown to be friendly, heroic and selfless, and then is suddenly laughing evilly and eagerly killing their former friends, it can be a bit off-putting to viewers.
Note this trope is not to be used any
time a character suddenly goes from a paragon of righteousness to a patron of wickedness. Brainwashing and mind-control automatically make this trope not applicable. This trope is when a character willingly
turns from good to evil, but their reasons for doing so are poorly explained or are just poor reasons, or the shift of personality from good to evil is so severe that the change in personality is difficult to take seriously.
Compare Shocking Swerve
, when a plot twist is poorly foreshadowed, and Sudden Sequel Heel Syndrome
, when a heroic character in the original product turns evil in a sequel.