Possessed or Brainwashed

Heroes have been known to get possessed, in which demons or spirits goes inside the heroes' bodies to control them for hurting a friend, killing someone, saying or warning something to a person they knew or a different person, or kill the possessed heroes from within, or because they just want a body to hide inside of. Sometimes, heroes want the demon or ghost to possessed them so they can have a better life or get powers or revenge.

Meanwhile, heroes can also get brainwashed when a villain lies to the hero about losing a friend, home, or family to make them their henchman. This could also be caused by the heroes feeling betrayed by their family and friends and them joining the villain because they are the last person they turn to for "help". Another cause of brainwashing is, of course, hypnosis.

Cartoons/Anime

 * Bart Simpson becomes hypnotized by his archnemesis Sideshow Bob, and to kill Krusty the Clown.
 * Ashi gets possessed by Aku when he reveals that he is her biological father.
 * Bumblebee gets possessed by Megatron, becoming his minion and reactivating Megatron.
 * Princess Luna has this, she though Princess Celestia was getting more of the attention of being princess which leads her jealously turns her into Nightmare Moon.
 * Officer Jenny was once controlled by an evil Malamar and became Madame X.
 * Twilight Sparkle has been possessed by the magic in her device, which makes her turn into Midnight Sparkle, but with her rampage this was Principal Cinch fault because she wants to cheat and win the games.
 * Styigan was possessed by the darkness "Pony of Shadows" because of hatred and anger and revenge. The reason why because of StarSwirl and the old pillars of Equestria.
 * Yami Yugi gets possessed by the Orichalcos by drawing its card. Yami Yugi became ruthless, savage, and so brutal to his monsters, and also want his enemies dead.
 * Fugitoid gets possessed by Overmind.
 * Kim Possible is mind-controlled by Dr. Drakken into becoming his minion.
 * Danny Phantom has been hypnotized by Freakshow's staff, which leads Danny Phantom and the other kind ghosts become minions to do Freakshows bidding by stealing jewels and money. Also he makes Danny Phantom and his minions to do his bidding to attack someone too.
 * Gumball gets possessed by a spirit called "Jealousy", which it makes him jealous over Leslie because he thinks Penny and Leslie are dating.
 * Vegeta has this, he yells at Goku saying that he should be at the World Martial Arts Tournament defeating Goku, which makes Dabura cancel the match and tells Babidi about Vegeta having an evil side and telling the others about the new recruit as Babidi thinks this is a brilliant idea in order to revive Majin Buu, and Babidi uses his magic to mind control Vegeta and transform him into Majin Vegeta.
 * Ryo Bakura gets possessed by an evil entity living in the millennium ring. Which is in fact Zorc himself, but he is just called Yami Bakura.
 * K.O. was blackmailed by the Shadowy Figure into becoming T.K.O.

Live-Action/Animated Films

 * Harry Potter is possessed by Lord Voldemort.
 * Mowgli and later Shanti are hypnotized by a hungry serpent named Kaa.
 * King Richard was hypnotized by Sir Hiss going into crusades that his brother Prince John takes over his throne.
 * Kent Mcoy gets possessed by the Cloyne. So the Cloyne can eat five children every winter day, and when all five are aten he will become normal again.
 * Optimus Prime is brainwashed into Nemesis Prime by the Cybertronnian sorceress Quintessa in Transformers: The Last Knight.
 * All of the Supers, including Helen, Bob, and Lucius, were hypnotized by Evelyn Deavor's hypno-goggles, so Evelyn can use them in wicked plan of making supers illegal forever.

Comics

 * The shadow ghosts were looking for an easily target on the mane 6 looking in their dreams for weakness or nobody wants them. Rarity, however, was dreaming about her friends that they don't want her anymore, which leads the shadow creatures to turn her into their queen, Nightmare Rarity.

Videogames

 * When Cynder was an egg, Malefor's goons took her in and Malefor made her his servant by corrupting her with his magic. After being freed by Spyro, she gets corrupted again when she is going to fight her master, which leads her to attack Spryo.
 * Spryo gets corrupted in Eternal Dawn when he is about to fight Ghaul only to get knocked to where Malefor makes dragons evil. Spryo became cold blooded pin his dark form. Spyro falls into the dark side a second time in Dawn of the Dragons because his mentor sacrificed himself to save him and Cynder, which leads him to get angry which Cynder calmed him down.
 * In the Dragon Ball Xenorverse games, the main heroes get possessed by villains a lot.
 * Santino Marella and later Finlay is possessed by The Undertaker because of the Mystic Urn's magic was cast on them in result of becoming an zombie with no hope of talking.

Others

 * Jaden gets possessed by The Supreme King because the reason why because of his feelings and trust of his friends and losing them.
 * Ichigo Kurosaki has two Zanpakutos inside of him, one is a good one and the other is an evil one. The evil one is Ichigo inner hollow. He thinks that Ichigo is the king while he is a horse. But when he posses him, he loves going for a fight and killing.
 * Shippo
 * Riku is possessed by Ansem, Seeker of Darkness.
 * Delia Ketchum has been hypnotized by Entei into believing she is Molly Hale's mother.
 * Ash Ketchum
 * Pikachu
 * Ginny Weasley
 * Honker Muddlefoot
 * Herb Muddlefoot
 * Binkie Muddlefoot
 * Launchpad McQuack
 * Darkwing Duck
 * Regan MacNeil gets possessed by Paruru until the exorcism.
 * Tommy Oliver has been by Rita Repulsa and transformed into an evil green ranger.
 * Jason Lee Scott and Kimberly Hart are being by the evil Maligore after they are captured by Divatox.
 * Shane, Dustin and Tori are been by Lothor who disguised as Sensei and ordered them to attack the Dino Thunder Rangers.
 * Cloe and Yasmin gets corrupted when the black pixies caught them and put black pixie dust on them turning into one of them.
 * Carolyn Perrison gets possessed by Bathsheba, after she tr to kill one of Carolyn daughters for a sacerfice for the devil.

Quotes
"My maker, I do."

- Optimus Prime becoming Nemesis Prime.

"Hello, Abby."

- Rowan North before his ghost pops out of the sink and possessed Abby Yates.

"(Ratchet: Omega, what are you doing?! Don't you recognize your old friend?!) The question should be "Don't you recognize your old enemy?" (Optimus Prime: Megatron?!) Ah, then you do remember my name. Pity I can't remember yours."

- Megatron brainwashing Omega Supreme.