There's a saying that "power corrupts absolutely," and more often than not, that turns out to be true. For many characters in isekai anime, the great power they wield is used responsibly, but sadly some succumb to the feeling of superiority that it gives them and the temptation to use it for their own desires.
Power comes in many forms, from physical to authoritative, and with them, there are many ways in which it can get the better of even the purest of heart. With some of the extreme levels of power these characters are given, it's not too surprising that some of them let it go to their heads.
10 Subaru's Head Swelled When He Assumed He Was The Hero (Re:Zero)
"Powerful" is not a word one usually uses to describe Re:Zero's Subaru Natsuki, given his constant state of death and rebirth. Despite that, when push came to shove, Subaru was able to use what little he had to accomplish good things, but this gave him a swelled head as he assumed he was a hero.
Subaru, high off the feeling of his few triumphs, started to make a fool out of himself and embarrassed Emilia in front of the royals by declaring himself her knight, which leads him to get thrashed by Julius. In the end, his mistakes cause Emilia to temporarily leave him behind which started him down the path to eventually redeem himself.
9 Taichi Rushed Into Battles Without Thinking Due To His Overconfidence (Isekai Cheat Magician)
As the self-proclaimed "Cheat Magician," Taichi Nishimura started out with a huge advantage over his opponents with his extremely broken power level. Since Taichi could basically take on anyone while barely trying, this made him more than a little overconfident as he rushed into battles without thinking.
His brute force strategy worked out pretty well for him at first, but it couldn't last. During a fierce battle with a two-headed dragon, Taichi's friend Anastasia is mortally wounded and he's unable to save her. After her death, he resolves to use his power more responsibly so he can protect the people he cares about.
8 Saddler Uses His Position As A Holy Knight To Justify His Sins (How NOT to Summon a Demon Lord)
The title of "Holy Knight" invokes the idea of a heroic warrior who uses their strength to nobly protect others, but this is not the case for Saddler of the Lyferia Church. While he may act like a kind soul who loves all people, in truth, Saddler uses his position as a Holy Knight to justify the many horrible actions he and the Church itself take.
Saddler abuses his authority as a Knight to arrest anyone he feels like (mostly non-human races) on false charges of being Demon Lord Worshippers, taking great joy into torturing them until they "confess" or just straight up killing them and calling it "forgiveness." Saddler is nothing more than a violent and delusional villain.
7 The Not-So-Cautious Seiya Watched As The Demon Lord Killed All Of His Allies (Cautious Hero)
Seiya Ryuuguuin is the hero who gets summoned by Ristarte who is overpowered and overly cautious. Surprisingly though, Seiya did not start out as pathologically cautious as he is currently, as it was the result of a previous and much more tragic adventure.
During Seiya's original isekai adventure, he was a lot more reckless, lunged into battle without preparation, and managed to keep winning through raw power alone—that is, until they faced the Demon Lord. After prematurely celebrating, the Demon Lord rises again and kills all of Seiya's allies, forcing him to watch the consequences of his overconfidence. While he lost the memories of these events, the emotional scars it left would make him the overly cautious hero he is today.
6 Quinella's Used Her Absolute Authority To Keep Herself Young & Powerful (Sword Art Online Alicization)
The Administrator and ruler of Underworld's Human Empire, Quinella's lust for power began when she discovered unique Sacred Arts that nobody else knew about to make herself incredibly strong. Tricking the people into believing she was blessed by the gods, Quinella quickly rose to power and created the Central Cathedral and the Axiom Church.
Eventually gaining authorities that only the Cardinal System had access to, she took control of Underworld itself, becoming less and less human in the process. Quinella became mad with her god-like authority the moment she got it, using it to keep herself eternally young and powerful while regulating the world.
5 Sara's Obsession With Amir Gaoul Drove Her To Want More Of It (Kiba)
Sara was originally transported from her homeworld to the country of Templer, as she was believed to have been chosen by the spirit, Amir Gaoul. Sara would undergo rigorous training to become a shard caster who could wield the spirit, but fate had different plans. As it turned out though, it wasn't Sara who Amir Gaoul chose, but rather her son Zed.
Returned to her own world and practically catatonic, Sara became obsessed with Amir Gaoul's power and wanted nothing else but to experience it again, even if she had to kill her own son to do it.
4 Tai Accidentally Unleashes SkullGreymon (Digimon Adventure)
In the original Digimon Adventure, Tai was often known for jumping headfirst into danger without thinking after being transported to the Digital World with the other DigiDestined. While this ability to not flinch in tough situations is partly what got him the Crest of Courage, sometimes this reckless attitude got him and others into trouble.
After being the first DigiDestined to gain his crest, he got cocky and tried to force Greymon to Digivolve, believing they could handle things on their own. However, Tai would realize his mistake pretty fast when his actions resulted in Greymon turning into the monstrous and out-of-control SkullGreymon.
3 Kazuma Temporarily Made His Comrades Do Whatever He Wanted By Wearing A Deadly Choker (KonoSuba)
Lazy, self-centered, and pretty unreliable nine times out of ten, Kazuma can be a real piece of work. He's many things, though "powerful" isn't one of them. However, there was one time when this unlucky jerk managed to get some level of authority and, of course, it immediately went to his head.
After accidentally putting on a choker that would suffocate him to death unless he fulfilled his wish, Kazuma took advantage of the power of guilt, forcing his comrades to do whatever he wanted, no matter how weird or skeevy it was. Ironically the collar came off at the last second, and Kazuma brutally got his butt kicked for all the trouble he caused.
2 The Many Mistakes of Ren, Itsuki, & Motoyasu Had Larger Consequences Than They Thought (The Rising Of The Shield Hero)
Despite the hardships and challenges that the Naofumi went through as the Shield Hero, he managed to rise above it all and become a real hero. The same cannot be said for the other three Cardinal Heroes, Ren, Itsuki, and Motoyasu, who let the power, fame, and sense of entitlement of their status get the better of them.
Due to their preconceived notions through fantasy RPGs and living out their own power fantasies, the three didn't recognize that their careless actions had consequences and over the course of the series, Naofumi discovers how much damage their ignorance actually caused the world. It was so bad that the Three Heroes Church planned to assassinate them all, deeming them incompetent and false heroes.
1 Ainz Ooal Gown Flexed His "Supreme Power" (Overlord)
For the better half of Overlord, Ainz Ooal Gown maintained a slow and steady plan of trying to understand the new world he and the Tomb of Nazarick had wound up in. But as time went on, Ainz started to lose interest in this approach and decided to finally start using his immense power to conquer this new world.
Allying himself with the Baharuth Empire, Ainz reveals his identity as undead and proceeds to flex his magical power as he utterly wrecks the Re-Estize Kingdom's army without even breaking a sweat. He then demands that everyone "applaud his supreme power." It was here that Ainz had crossed the point of no return.