Parental Betrayal

Sometimes the villain in a work hits much closer to home than the hero would expect - it's their parent. Yes, the parent who we've seen throughout the movie, book or show, doing their usual parent stuff and being seemingly a good guy, only to turn out to be evil all along. Can be very shocking if pulled off right.

This also refers to when a character's previously unseen parent is revealed to be evil.

Not the same as Luke, I Am Your Father, which is basically the inverse - the villain (who was previously shown) is suddenly revealed to be the parent, rather than the other way around.

See also Parental Abandonment. Offing the Offspring can either ensue after this, or be the way this trope is revealed.

Anime/Manga

 * While on the extremely low end of this trope, Beyblade has Judy Tate, who sandbags her visiting son Max Mizuhara and his friends into being test subjects for the corporation she works at, mostly so they can obtain data to beat the Bladebreakers. Later in the series, she does aid them, and is not truly a big bad. Also, when her son's victory  she smiles and simply starts over, and wishes her boy well. But she sees no conflict in using her son this way, openly mocks his philosophy (while half-allowing for the possibility of it) until beaten, and even chides his teammates for not seeing the deceptive methods she used as 'a friendly match', and the only ones who call her out are the other people at her corporation, who wonder if her loyalty to Max will affect their research.
 * There would be no Far Side routes if this wasn't the case for Shiki/Akiha in Tsukihime.

Comic Books

 * This is the premise of Marvel's Runaways. Children learn that their parents are supervillains, and run away from home, fearing they would otherwise grow up to be like them.
 * The Batman R.I.P. comics storyline toys with the possibility that the villain responsible for the defining tragedy in Bruce Wayne's life was . The conclusion of the story kinda seems to confirm that someone was just messing with him in the end.
 * Jason Todd, as you know, had horrible luck with his mom too. After finding out that his mother who raised him was not his biological mother, Jason sets out across the globe searching for his birth mother. Eventually Jason is reunited with her; however Jason's mother is evil. There is really no other way to describe a woman who would so willingly and suddenly betray her fifteen year old son to the Joker and actually stand there watching him be beaten brutally to near death with a crow bar, all the while looking none too fazed. After all this happens, the Joker betrays her, ties her up and leaves her and Jason in a room with a bomb. Jason, injured though he is, tries to save the mother who just betrayed him. He cuts the ropes binding her and together they make it to the door. Naturally the door is locked. The bomb explodes, Jason and his mother die. He got better.
 * And Bruce's son Damian hasn't been much luckier. He wasn't pleased at all when his mother Talia willingly allowed Slade to control his body by means of implanting a device in his spine. And when he supported Dick and Bruce's ideals rather than his Talia's,  Jerkass Woobie indeed.

Film

 * In Hot Fuzz, it turns out the mastermind behind all the 'accidents' is.
 * Nathan in Repo the Genetic Opera, although the reveal comes immediately after the first scene with him.

Literature

 * In the Grey Griffins books, Max's father turns out to be evil and wishes to use Max to gain access to a world-destroying artifact. He has no qualms about killing his own son and his son's friends - genuinely shocking to Max, since he doesn't get along with his mom, and had always preferred his dad.
 * In Canadian teen adventure novel, Jack's Knife, a girl's father is involved in some criminal operation. He binds and gags his own daughter, which freaks out the main character (who is thinking "my god! He's your own daughter!"). When the man's partner suggests doing away with the kids, he is nice enough to suggest that doing so would be a bad idea ("we don't want to be wanted for murder as well"), but it's left unclear whether this is for selfish reasons, or to spare his daughter's life.

Live Action TV

 * Twenty Four: was a Big Bad, the Man behind the Man  behind the Man  behind the Man (attempted assassination of the Russian President). [More behind the Mans possible, I can't keep track.] Attempted to kill his son.
 * Played straight and then inverted in Angel's final season episode 'Lineage'. Roger Wyndam-Pryce, Wesley's father. It turns out he intends to steal Angel's free will. Then it turns out he was a robot (which was shot by Wesley BEFORE the revelation).
 * Depending on who you ask, this may be present in Firefly in the form of River and Simon Tam's parents, whose callous disregard for River's plight at the Academy may have been due to foreknowledge of what the Academy was doing to her.

Mythology

 * Gaia of Greek Mythology did this to three generations' worth of her divine progeny after they pissed her off by empowering the next generation. Her first son Uranus who was also her first consort was jealous and fearful of their children the Titans and trapped them inside Gaia. A pissed off Gaia forged a huge sickle and gave it to her youngest son Cronus and had him castrate Uranus with it. Cronus eventually turned out to be Not So Different from Uranus and trapped his non-Titanic siblings, the Hechatonchaires and the Cyclopses, inside Gaia since he feared their power. Being Genre Savvy, Cronus knew that Gaia would try to turn his own children against him in revenge, so he ate his own children after they were born. A pissed off Gaia conspired with Cronus' wife Rhea to save the last child Zeus, and raised him to be a Laser Guided Tykebomb against his father. After Zeus and his fellow Olympians eventually prevailed, they stuck the Titans who refused to surrender to them in the deepest pits of Tartarus, which upset Gaia since she didn't want her children to suffer such a horrible punishment. A pissed off Gaia then sent various giant monsters such as her youngest and strongest offspring Typhon to overthrow the Olympians. This chain of betrayal ended when Zeus and the Olympians defeated the threats sent by Gaia, proving that they had surpassed her.

New Media

 * 's dad and 's mom in Lonely Girl 15 are both examples.

Video Games

 * In Mass Effect, Wrex eventually explains that he was a leader of his people back on his homeworld following the Krogan Rebellions. He was advocating peace so that the Krogan could rebuild, but his father, Jarrod, was a rival warlord pushing for war. Wrex and Jarrod agreed to meet at neutral ground to discuss their differences. Being krogan, though, it could only end one way. Jarrod's troops managed to wipe out Wrex's followers, forcing him to leave the planet both for his own survival and because he was disgusted with his own species' self-destructive nature, but not before Wrex managed to kill his father for the betrayal.
 * The Silent Hill series does this several times:
 * In the original Silent Hill, Dahlia Gillespie set her own daughter on fire, but magically kept her alive, to force the girl to use her magic powers for Dahlia's own sinister purposes. This is referenced in Silent Hill 3 and Silent Hill Origins.
 * In Silent Hill Homecoming,
 * In Diablo III

Western Animation

 * In The Legend of Korra it turns out that  is an Equalist. The traditional "We Can Rule Together" spiel is given, but the child violently refuses.