WARNING: The following contains spoilers for My Hero Academia Chapter #310, "Masters And Pupil,” by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
Chapter #310, “Masters And Pupil,” of My Hero Academia revisits the discussion Deku had with all the wielders of One For All in the mystery realm. It turns out Yoichi, All For One’s brother, was saved by the second and third uses of One For All, even though he had ties to their mortal enemy. The true power of One For All was awakened by the pair saving Yoichi, and that same energy translates now into Deku’s will to save society from All For One, and potentially Shigaraki, as well.
The reason the second and third users are still on the fence about whether they can support Deku is that they don’t believe that the young hero can accomplish the impossible. Deku wants to save Shigaraki from the clutches of All For One, which means he wants to help the 'enemy.' The current rise of villain activity is similar to the era when All For One had control over society. Therefore, the second and third users do not want to see another rise of villains with All For One at the helm. But Yoichi promises that this time around, that won’t be the case.
When the second user first came into contact with Yoichi, he had the option to kill him, since he shared All For One's lineage. Instead, he saved him. That was the defining moment in which One For All's true potential was ignited.
Deku’s resolve to save Shigarki resonated with Yoichi. It not only reminded him of the time the second wielder saved him, but it also meant that his adopted nephew, Tomura, still had the potential of being saved from his brother, All For One. Since Yoichi has a strong sense of justice, he could be hoping to save his older brother, but as All For One’s body has been deteriorating over the years, if he doesn’t get a new one, All For One’s time will run out. And even if All For One passes away, all his malice and hate has already been transferred to Shigaraki.
All For One is aware of the time he has left in his current body, but might not fully understand the depth of development One For All has gone through after being transferred to so many worthy individuals. The wielders that have been revealed have all had comparatively weak Quirks that were not meant for saving people. But that only fueled their need to help those who were completely powerless. A hero is not the one with the strongest ability, it's the one with the strongest will to save anyone, no matter the odds.
The conversation Deku has with a civilian he saves from scared bystanders at the start of Chapter #310 only verifies his unweaving will to makes things right. The civilian wishes for things to be back to normal, and Deku confidently confirms that he’ll make sure that happens. His level of confidence is greater than it has ever been, but that’s because he knows there is no room for failure. He also has all the past wielders supporting him now, not to mention the top three Pro Heroes watching his back. All that's left is for Deku to truly believe he can save everyone in order to unlock One For All’s full power.
It will be interesting to see if Deku’s sense of power shifts if he fails to save even just one person. One For All's might not necessarily weaken, but the power could lose its balance and wind up hurting Deku as it did in the past. Deku is going to have to constantly keep himself in check, and arrive at the realization that one person can’t save everyone. Then again, maybe that is the reason why Deku will defeat All For One in the end, knowing that doing so may be the closest he can come to bringing peace to all.