Warning: contains spoilers for My Hero Academia chapter #355!
The ultimate villain in My Hero Academia was just defeated by a hero he already killed. All For One saw his own powers revolt against him as a consequence of absorbing Star and Stripe's quirk, New Order, which means that America's no.1 hero managed to defeat the villain from beyond the grave.
The most powerful superpower (called "quirks" in the series) shown in My Hero Academia is without a doubt All For One, possessed by the eponymous villain and by his "vessel" Tomura Shigaraki, which allows them to steal other people's quirks with a simple touch. Over the years, All For One stockpiled a great number of powers without issues, until he tried to absorb Star and Stripe (Cathleen Bate)'s New Order, another incredible quirk. New Order allows its user to "set a rule" onto their surroundings, letting them bestow new properties onto themselves and the world around them. Shortly before dying, Cathleen used this ability on her own quirk, basically turning it into a virus for All For One.
Star and Stripe arrived in Japan after the country fell into chaos due to the actions of All For One and Shigaraki. Star and Shigaraki immediately fought each other, and the American hero sadly fell in battle. However, with her last breath, she set a final rule: that New Order would revolt against other quirks. Once absorbed by Shigaraki, New Order starts destroying the other quirks contained in All For One, causing great damage before Tomura gets rid of it. It seems that New Order has finished its job, taking many quirks away from All For One and damaging Tomura enough to give the other heroes time to prepare for the final battle, but chapter #355 reveals that Star and Stripe did much more than that. All For One's main body is suffering from the aftereffects of the attack, which brought to light the "wills" of the people he absorbed quirks from, making them rebel and exposing All For One to the heroes' attack.
At first look, All For One's quirk seems to have no restrictions or limits. The user can steal as many powers as they want and freely redistribute them. However, the villain once mentioned having frequent nightmares where the consciousnesses of the people he stole Quirks from appeared before him. Quirks can, indeed, preserve the will of the original owner, and All For One should be well aware of this because this is how he is taking over the body of Shigaraki, with a vestige of his personality contained in the quirk. Clearly, All For One believed he had complete control over the quirks he absorbed because his will is so much stronger than the original owners, who he disdainfully calls "weaklings." New Order's invasion, however, had unexpected effects. It did not only affect Shigaraki's body but All For One's too, thanks to the connection shared through the duplicate quirk.
All For One is a serial user of others, so it's perfectly fitting that he's weakened by the sins of his past. All For One is literally paying for every person whose quirk he stole, with more damage taken in direct proportion to his misdeeds. If the next chapter of My Hero Academia shows that the heroes' attack really brings down the villain, it will be thanks to both Star and Stripe and All For One himself, whose past victories are finally turning to ash in the perfect ending for the manipulative villain.