Whether in reality or anime, the concepts of good and evil are never black and white. One's perspective decides influences what they deem good and evil. Anime heavily elaborates on this, and many series blur the lines between good and evil. When analyzing those stories, it's clear several anime characters have crossed over to the dark side without even realizing it.
Whether blinded by their singled-minded focus or simply too distracted to realize the change withing them, many anime characters risk becoming the very thing they're fighting against. In an even darker twist of fate, other protagonists know full well that they are embracing evil, but feel the end results will justify their actions.
10 Ken Kaneki (Tokyo Ghoul)
Tokyo Ghoul makes fans question their morality. Between the Ghouls who eat humans to survive, and the CCG who hunt Ghouls them down, fans don't know which side to empathize with. Ken Kaneki experienced both sides of society, increasing his understanding of the Ghouls and the CCG.
After losing his memory, Kaneki began to work for the CCG under the name Haise Sasaki. Despite understanding the Ghouls' plight and their fear of being hunted down just for existing, Kaneki sided with the CCG. While he may not have remembered the Ghoul he used to be, he still turned against his own kind.
9 Satsuki Kiryuin (Kill La Kill!)
The revelation that Satsuki was Ryuko's older sister in Kill La Kill!, changed everything in the series. Initially, Satsuki portrayed herself as a strict, pragmatic leader who ruled Honnoji Academy with an iron fist. Satsuki tried to crush Ryuko, using the entire school in her attempts to cast her out of the academy.
The reason behind Satsuki's grudge became apparent toward the end of Kill la Kill!. According to Satsuki, she purposefully gave Ryuko a hard time to test how strong she was. Knowing that Ryuko would need a powerful ally to stop their mother, Satsuki deliberately played the villain to stage a coup against their mother down the road.
8 Aleister Crowley (Toaru Majutsu No Index)
Aleister Crowley, much like his real-life counterpart, is a complex character. In Toaru Majutsu No Index, Aleister became a desperate magician after the Magic Gods of his universe caused the death of his daughter. Her death was the catalyst for Aleister's decision to erase magic forever.
To carry this out, Aleister orchestrated several events in Academy City that made others suffer. Aleister funded experiments that tortured children. He allowed the death of innocents and manipulated people's lives to benefit his goals. As Aleister himself admitted, he'll do anything to wipe magic from existence.
7 Mai Tennouji (Shy)
Mai is an extreme example of what happens when society forces good people to do bad things. In Shy, Ai and Mai were twin sisters who belonged to a shinobi clan. Unknown to Ai, the village would send Mai on many missions to assassinate their enemies. This conflicted with Mai's innate sweet nature. As a result, the more people Mai killed, the more miserable she felt.
After the deaths of her best friends, Mai adopted a new persona to fight evil. Under her new name, Hollow, Mai cut down her sister and fled the village after years of having her guilt weigh upon her. Closing off her heart to others, Mai began fighting against humanity.
6 Tsukasa (Dr. Stone)
As chaotic and unpredictable as Tsukasa seemed in Dr. Stone, his logic made sense. The second person freed from petrification, audiences thought Tsukasa would stand alongside Senku to save the world. However, problems arose when Tsukasa suggested destroying the people in society who made life difficult for those beneath them.
Senku didn't share Tsukasa's point of view. The two fell out over this fundamental difference and went to war with one another. While Tsukasa's idea made sense logically, Senku and his army understood they didn't have the right to dictate life or death like Tsukasa wanted.
5 Lady Nagant (My Hero Academia)
Lady Nagant became Deku's latest opponent during the 6th season of My Hero Academia. Like Deku, she aspired to be a great hero who helped others and inspired hope. The government ruined her dreams when they tasked her with covert ops missions that forced her to assassinate villains from the shadows.
Lady Nagant's realization that the public would never know about the corruption of society warped her ideals. She fell from grace and became an assassin for hire. Ironically, Lady Nagant's job of killing villains led to her becoming one of them. It took Deku defeating Lady Nagant for her to realize that she could turn her life around and choose a heroic path.
4 Keyaru (Redo Of Healer)
Redo Of Healer will go down as one of the most extreme anime ever produced. What Keyaru's former teammates subjected him to goes beyond inhumane. After enduring years of torment, Keyaru decided to enact vengeance on his abusers. Tragically, he had no problem stooping down to their level and even below it.
One by one, Keyaru took down his old teammates in the most brutal ways imaginable, his thirst for vengeance leading him to commit deeds worse than those who'd wronged him. In the end, Keyaru's pursuit led to him becoming the very thing he fought against.
3 Nana Hiiragi (Talentless Nana)
Deceptive to the end, Nana systematically killed numerous classmates in her school under the guise of being their friend. From Nana's perspective, her super-powered classmates were enemies of humanity and threatened society. This turned out to be a lie created by the government to punish Nana's father for going against them.
When Nana found the government had manipulated her into murdering innocent kids, it mortified her. Understanding that she could never fully atone for her crimes, Nana switched allegiances to save their lives.
2 Kiritsugu Emiya (Fate/Zero)
In the Fate/Zero series, Kiritsugu suffered throughout most of his life. Forced to kill his father and adoptive mother, he strived to become a hero of justice who would eliminate all evil in the world. Upon killing hundreds, if not thousands of people, Kiritsugu eventually realized that the fight against corruption lasts forever.
Killing one villain spawns two more in their place. Kiritsugu could never call himself a hero if evil always remained. This epiphany led to Kiritsugu living his life like those he would kill. With no room for hope or justice, Kiritsugu closed himself off from feeling sympathy toward others.
1 Pain (Naruto)
There likely is no grander example of heroes turned villains than Pain. Initially creating the Akatsuki as a peace-keeping organization, Pain used violence to invoke an age of peace in the shinobi world.
Operating under the belief that subjugation through fear is the only method for serenity, Pain devastated villages to capture the Jinchuriki, ruining thousands of lives in the name of peace. As time passed, Pain forgot what it meant to keep people safe. His sense of morality did a complete heel turn as he witnessed more and more bloodshed and violence.