My Hero Academia's main antagonist, All For One, has long appeared to be a classic villain hellbent on committing the most vile atrocities against humankind. While it's true that he is genuinely relishing in acting as an Immortal Demon Lord and terrorizing humanity for the past century, that doesn't mean he is incapable of some change in personality.
All For One has long reigned as the vile ruler of Japan's societal underbelly, controlling the strings all by himself. With an earth-shaking degree of confidence in all his actions, he has appeared to be working just fine on his own. That being said, suddenly changing his plans to include a protégé -- Tomura Shigaraki -- was a critical change in character.
All For One's Egoism and Assumed Immortality in My Hero Academia
As My Hero Academia has progressed, viewers have come to understand All For One's worst and most irredeemable traits. Aside from his simpleminded inclination toward immorality, he is the perfect example of an arrogant megalomaniac with a concerning level of self-importance that goes beyond narcissism. As All For One grew in power, he began to see himself as Godlike, with everyone else below him. This perfectly fit his ideal image as an Immortal Demon Lord -- but his arrogance grew even stronger when he attained the Quirk known as Life Force.
Life Force doubles its user's lifespan, which further boosted All For One's ego. It's also a great asset to his vile campaign as he's been able to acquire more support and strength over the decades. With the power of any Quirk he desired, an obsessive following along with vast resources and his inability to die, All For One appeared to be unstoppable. This merely added to his lackadaisical reaction to anyone who dared to oppose him.
All For One had no competitor within the first several decades of his control over Japan -- until his younger brother Yoichi joined forces with a rebel group fighting to end his rule. Unlike other opponents, Yoichi and the rebel leaders became united with the One For All Quirk, which, at the time, showed signs of being the only weapon that could defeat All For One. As decades passed, All For One watched as it was passed from one protégé to the next, but didn't think about taking on a student himself. Being in his prime and virtually incapable of dying, he saw no need to pass the torch on to someone new. However, after nearly meeting his end against All Might, All For One's high opinion of himself changed.
All For One Finally Recognized He Alone Isn't Enough
Although All For One and All Might's clash was essentially a draw, it left both with nearly fatal wounds. In the century that All For One has been alive, no one else had ever scathed him so much, so that battle left the villain with a new perspective. Seeing how much stronger One For All had become as it was passed through the generations, All For One finally saw the benefit of having a protégé to aid in his work. However, taking in Tomura Shigaraki as his student was never just a change in tactical strategy.
All For One's relationship with Shigaraki is surprisingly unique. Granted, his manipulation of the boy isn't new; he proved in Season 6 of My Hero Academia that he only aims to use Shigaraki like he does everyone else. There is something notable, however, about his changed attitude toward their relationship that reflects his stance on mortality. Without sounding condescending, All For One does state that he wishes to pass a legacy along to someone else.
When Shigaraki violently threatens his master for forcefully taking over his body, All For One's reaction isn't his typical ridicule or criticism. Rather, he warmly accepts Shigaraki killing him so long as that means his student takes over his legacy. For once, All For One is not thinking only of himself or his rule as a Godlike being; he accepts mortality. These are reactions and traits he never showed a century ago and, although the change took many decades to occur, his relationship and mentorship is a sign that All For One is preparing for a world without him in it.
The Truth About All For One's Life Force
There is a sense of spite that All For One has in regard to taking Shigaraki in particular as his student. The latter is the grandson of Nana Shimura, the seventh One For All user and All Might's mentor. All For One certainly enjoys the look of torment on All Might's face when he realizes who Shigaraki is, but that doesn't explain why someone so egotistical would welcome death at the hands of his student. It's highly unlikely this is out of a sense of genuine care for the boy, as All For One still hasn't shown remorse for the pain he's put Shigaraki through in My Hero Academia. What this appears to be is All For One's readiness to leave a legacy.
This change in All For One's opinion on his own potential death is arguably something that formed over the past decade or so. In reality, he is not immortal and shows that he's accepting his mortality. Despite the benefits of the aforementioned Life Force Quirk, it's been proven that once it is taken away, the user ages rapidly to reach the age that they should be at -- which is what happened to Dr. Kyudai Garaki when the Quirk was briefly taken away by Eraserhead.
With more evidence that Life Force is simply an extension of life expectancy, it makes even more sense for All For One to accept that he will eventually die and why he wishes to leave a legacy for Shigaraki. Though his ideals and motivations are still certainly rooted in his narcissism, it can't be said that All For One hasn't changed. My Hero Academia's ultimate villain might still think highly of himself, but even he accepts that he isn't above death.