Fallen Hero/Playing With

Basic Trope: This person was once a hero until something very bad occurred.

Straight: Bob was once a Knight Errant until he found that his Doomed Hometown was burned while he was away. He's lived as a lowly drunkard ever since.
Exaggerated: Alice was once The Messiah with a Thou Shalt Not Kill policy. Then, the court made an agreement with Emperor Evulz and branded her party as traitors. After she escapes from the siege alone, Alice found that her little sister was killed by the villagers, who feared that the army might attack them as well. These events turned her into a Complete Monster to which Evulz can't even be compared.
Justified: The events topple the very ideals held by the character. Traumatized, her mental state is irrevocably shattered.
Inverted: The Atoner
Subverted:
- Bob may be a drunkard, but he will agree to help if The Hero needs him.
- Bob was a Fake Ultimate Hero and was never a good person to begin with.
- Alice is a Noble Demon and much more likeable when compared to other villains.
Double Subverted: Alice has Pet the Dog moments every so often, but decides that her deeds are Necessarily Evil and sticks to them.
Parodied: After the court forgot to invite her to the national ball, Alice decided to lead Evulz' troops to curbstomp the country before summoning Hell on earth.
Deconstructed: As a hero, who's been the best the world had to offer and decided being good was worthless, Bob is worse than any standard villain. Additionally, when he falls, he takes many others like him down with him, either because he has convinced them to agree with him or they are just disillusioned that their hero could do this.
Reconstructed: The idealistic Dean decided to make Bob remember what it's like to be Good, with good old Good-powered chainsaw fisticuffs to the jaw.
Zig Zagged: Alice was a hero until she was betrayed by her country and fell. However, it turns out that she still regularly performs heroic deeds and only appears to be evil. However, that turns out to be an act, and she is actually planning total destruction. But it turns out that she was a Well-Intentioned Extremist. Then another betrayal plunges her into true villainy.
Averted: Bob and Alice continue to hold their beliefs and become Failure Knights In Sour Armor.
Enforced: Despite all the evil acts, Alice is the fan favorite. So the producer decides to make her into a more sympathetic character by laying out her Start of Darkness.
Lampshaded: (to The Hero) "You're just like Alice. If it wasn't for that day..."
Invoked: See Exploited.
Exploited: Evulz' engineers Bob's/Alice's downfall to make him/her his champion.
Defied: The ghost of Alice's little sister chastises her as she begins considering taking the dark path, snapping Alice to her senses.
Discussed: ???
Conversed: ???
Played for Laughs: Aside from being Ungrateful Bastard s, everyone is also retarded enough to fall for reasoning that wouldn't convince an eskimo to buy ice. That being the case, Bob/Alice, the most Obviously Evil villain imaginable and being known world-wide for her moral downfall, manages to persuade everyone to start a war against each other.
Played for Drama: Bob/Alice takes delight on taking horrifying vengeance on his former friends, aware how agonizing they are finding his/her chain of retaliations.

Stop, turn back, don't go down the path of the Fallen Hero.