Warning: SPOILERS for My Hero Academia's Season 6, episode #133The My Hero Academia anime confirms that the series' greatest villain, All For One, has only one weakness, and it's incredibly surprising. To steal from Izuku Midoriya (Deku) the power he's been chasing, One For All, the villain needs a strong hatred, capable of overcoming the willpower of the nine successors of One For All. However, despite calling himself a Demon Lord, All For One apparently does not feel any hatred.
In episode #133 of My Hero Academia, "Hired Gun", the three top Pro Heroes, Endeavor, Hawks, and Best Jeanist, are working together to try to contain the villains who recently escaped from prisons all around the country. Discussing the truth behind the One For All Quirk, which they only recently discovered, Hawks says that he does not understand why All For One needed to raise and nurture Shigaraki to feel the strongest hatred possible, in order to steal One For All. Hawks rightfully points out that All For One should already feel great hatred for the successors of One For All, who have been keeping that power away from him for generations. However, Hawks doubts that All For One is capable of feeling hatred, or any other feelings at all.
All For One's Only Weakness Is His Own Lack Of Humanity
All For One is My Hero Academia's biggest villain, a man who, when Quirks first appeared, used his ability to steal these powers to plunge Japan into complete chaos. Generations of heroes tried to stop him, but the only power capable of opposing him is One For All, created by his brother Yoichi and passed down through the years and many successors. The eighth user, All Might, was able to defeat the self-styled Demon Lord twice, but this did not end his threat. All For One's plans were still in motion, and they hinged around the broken boy he found and raised as his "successor", Tomura Shigaraki.
All For One's plan is to foster Shigaraki's hatred so that it could become strong enough to overcome the will of One For All. Then, by giving his Quirk to Shigaraki, All For One would take over the boy's body and use it to steal One For All, completing his ascension to Demon Lord. Hawks's analysis of this plan points out that it only makes sense if All For One is, in fact, incapable of feeling hatred himself, as proved by the fact that he is always (wickedly) smiling. As Endeavor says, the villain "lacks a heart", which is actually a fascinating spin on the character. All For One is not simply "evil", he has completely shed his humanity, meaning he is no longer capable of any feelings outside his boundless ambition.
Ultimately, All For One's wickedness proved to be a double-edged sword, making him ruthless and capable of anything, but also stopping him from stealing One For All a long time ago and forcing him to enact a long-term plan that relies on Shigaraki. This is a surprising twist that adds more depth to My Hero Academia's main villain: All For One is power incarnated, but his only weakness comes from his own lack of humanity.
Season 6 of My Hero Academia is streaming on Crunchyroll.