Warning! Spoilers ahead for My Hero Academia chapter 326!
The hero killer Stain seems to have been perfectly content with watching the latest events of My Hero Academia unfold from the shadows (while exposing his tongue in signature fashion), but the rogue antihero just took action in the most unexpectedly shocking way by giving the former hero All Might a new sense of purpose.
Before his early retirement, All Might first began contending with excessive drawbacks that came from a rather extensive injury he sustained during his first pivotal fight against the villain All For One, making it difficult for the hero to maintain his beefed-up and chiseled super form. Failure to do so resulted in him reverting into a pathetically gaunt and haggard man who had to work quickly and hide before transforming in public so the people, who all put their faith in him, wouldn't see who he really was and lose hope. This was undoubtedly stressful for All Might but, ironically, his second run-in with All For One exacerbated his previous injury, further complicating his ability to stay in his superhero form. This required All Might to retire as a hero, relinquishing his coveted position as the people's main source of hope.
All Might reached a whole new low when his student and most fervent admirer Deku pushed him aside during his solo mission to take on All For One when the only thing All Might wanted was to encourage Deku to rest. It wasn't just the fact that he couldn't help his student that tore All Might down, but that the boy who once looked up to him and did everything he asked with a frightening ferocity now didn't even feel compelled to look him in the eye. All of these intense emotions and regrets come piling down on All Might in chapter 326 of My Hero Academia. A dejected All Might finds a torn part of Deku's mask lying on the ground near a vandalized All Might statue, which has a sign dangling on its neck mocking the hero's signature catchphrase that once brought hope to the hopeless: "I am not here!" When All Might tells himself that he's just dragging everybody down, Stain appears.
Although Stain later gives the impression that he knows the thin man is actually All Might, the hero killer feigns ignorance, expressing anger that this pathetic and wretched shell of a man could ever be the great number-one hero. After All Might unloads on Stain about how much he's failed, the hero killer forces All Might to look at a girl who has just begun cleaning his statue. Stain reveals she's the last civilian All Might saved and that she comes there to restore his statue every day even at the detriment to herself. He then says, "The flickering embers [All Might] left behind is nurtured by those select few who succumb neither to chilling rain nor howling wind."
Stain believes that a person can only call themselves a hero if their soul truly desires to be in service of others and that what truly made All Might a hero wasn't his quirk, but that he couldn't live any other way. What Stain says obviously moves All Might to tears, especially as he watches the woman restore his statue as she says, "I'm doing my best. You, too, okay?" Without Stain's interference, All Might would have continued to spiral into a deep depression. But now he has a purpose, thanks to Stain, a reality that's further exemplified by the chapter's ending scene where a sky that has been persistently cloudy for the last few installments of My Hero Academia finally clears above the outstretched arm of All Might's restored statue.