Oftentimes, anime tends to focus on an overarching antagonist who works as the protagonist's foil throughout the entire seires. There's typically a solid revenge story behind a regular person turning into a notorious villain, but the viewers seldom see a character who started out strong but had to bend to the will of fate.
Shows like Tokyo Ghoul, Naruto, and even Attack on Titan showcase strong characters who carry immense hate for these monsters who destroyed their lives, but surprisingly, they end up becoming the exact thing they hate just to carry out their ultimate motive. It's a form of cruel irony to see these characters become what they despised the most, with many anime fans enjoying this trope as a guilty pleasure.
Light Yagami Became the Very Criminal He Despised
Death Note is a classic series that remains a fan-favorite decades after its original release. Light Yagami is a character every viewer can relate to because it's about a person thinking they can never do "something" wrong, when in reality, everyone can go down that path. Light Yagami loathed criminals and wanted to eradicate them from existence for the betterment of society.
When he finds the Death Note, Light sees an opportunity to finally fulfill his motive of making a better world. He kills criminals one by one, but it's soon established that it's not one man's job to serve as judge, jury, and executioner.By the finale, it was Light Yagami who became a criminal and sacrificed innocents to protect his own skin.
Zoro Eventually Became a Pirate
Rorona Zoro from One Piece openly declared that he hated pirates when he met Luffy. He was famously known as the "Pirate-Hunter", who killed pirates for bounties. Most of the time, these characters end up becoming an inseparable part of the group they once loathed.
Zoro eventually switches sides when Luffy saves him, and he becomes the second member of the Straw Hat Pirates as a skilled swordsman. Vowing never to become one of them, Zoro ended up becoming an integral part of the Straw Hats and fought for the very pirates he once used to kill.
Mika Was Forced to Turn into A Vampire
Vampire territory is so gullible that characters can't help but get sucked into the whirlpool of "I'm not strong enough" and eventually become blood-sucking monsters to either save someone or survive. Seraph of the End is a great example of a character becoming the very monster they hate, but instead of transforming themselves to protect their loved ones or further their goals -- many throughout this series change themselves out of malice and cruelty.
Mika is an orphan, and the only family he's ever known are other children. While trying to escape the vampire-infested city, they are met with Ferid Bathory, who slaughters everyone except Mika. The helpless boy is then turned into a vampire by the Third Vampire Progenitor against his will. Not only did Mika have to endure trauma, but he had to live the rest of his life being a creature he dispised the most.
Kaneki Becomes a Horrifying Ghoul
Kaneki hated ghouls for consuming humans, and their very publicized violent nature. Initially, he had a very bright side to him and took a liking to Rize-san, a young woman who seemed interested in pursuing a romantic relationship with him. In an unfortunate turn of events, Rize is revealed to be a ghoul, taking Kaneki to an abandoned construction site and tried to feed on him.
Ken Kaneki is severely injured in the accident, which also kills Rize. Unable to survive on his own, there's no option but to implant Rize's organs into Kaneki's failing body, turning him into a half-ghoul. Kaneki hated ghouls, but now he had to do the very thing that disgusted him in the first place just to survive his new form.
Eren Yeager Made Dire Choices
If fans want to know the most typical example of becoming what the character hated, Attack on Titan's Eren Yaegar is a very recent and notable use of this trope. Eren survived a horrendous invasion by titans, witnessing his mother being eaten in a violent, grotesque manner. This inciting incident leads him to the Survey Corps with a personal mission to wipe titans from existence.
Eren soon realizes he has the power to turn into a titan, not just any titan, but a force so destructive that it would shake the world to its core. Even though Eren's goals were clear from the start that he could use his powers for good, he has to embrace the very thing that ruined his life.