It was to protect his brother.
That's what All for One always said during his rise to power. Not that he hadn't enjoyed what he did. Gaining the devotion of his followers, eliminating enemies, his devotees acting as extensions of his will. It was almost like the demon lord in that comic he and his brother had read!
Yet the need to protect his brother still took priority. To keep him safe from this increasingly dangerous world. Sure Yoichi complained, but he was always a whiner so that didn't matter. He was safe in that vault. All for One had even been gracious enough to give him a quirk, which (wasn't easy with how frail Yoichi was).
Then his brother was stolen.
All for One amassed more power to take him back. Yoichi was his. He'd protect him whether Yoichi liked it or not! All for One gathered every resource to pursue him.
Then Yoichi died.
All for One had tried so hard to protect his brother, now he was gone. All for One's energy, his spark, left him. His empire went from a pedestal to an anchor. The prospect of conquering the nation once brought him excitement, but now it only promised extra hassle. Running an empire was hard enough as it was!
All for One moved his empire more to the shadows, and let it and himself drift through the years. Occasionally he'd perform random acts of villainy to lift his spirits. An enemy gruesomely killed here, a massacre of the innocents there, and perhaps an atrocity seasoned with some cliché villainy. They gave him a brief buzz of pleasure. But sadly such things soon faded and he was left again with the chore of running an empire which was slowly decaying. It seemed there was no way out of this malaise.
Then his brother's kidnapper, the one with the scar, began leading strikes against All for One's crumbling empire. This man took his brother from him, he was the reason Yoichi was dead. All for One quickly mustered his forces to kill him. Yet All for One soon noticed something different about the man. The light of his quirk now looked like two. And the second looked suspiciously similar to Yoichi's quirk after All for One gave it to him.
The scarred man was also stronger now, and seemed to gain strength as he went on. Almost like he was…stockpiling it. Eventually All for One overcame him. Some of the man's allies briefly delayed All for One, but it changed nothing. Nothing would deter him. Even the sight of the scarred man's partner escaping wouldn't deter him. All for One would hunt him later.
All for One held out his hand, touched the scarred man's forehead, activating All for One. But it didn't work. What? Why couldn't he take it? One comment from the man about his "brother still not obeying" broke any self-control All for One had. His enemy died, taking any information he may have had with him.
All for One stewed over the incident for days. Why did his quirk resemble Yoichi's? And what had he meant by that last comment?
The scarred man's teammate with the ponytail took charge of All for One's enemies. His quirk now resembled not two, but three. What was going on? After several weeks of analyzing, going over every possible solution, it hit him.
Yoichi's quirk was a power stockpiling quirk.
Either it had stockpiled All for One, or it had merged with a heretofore unknown quirk Yoichi had, allowing it to be passed on to new users. First his brother had passed it to his scarred friend. Then it passed down to his ponytailed teammate.
All for One realized two things. One, that quirk was his. He'd given it to his brother out of the goodness of his heart! He was Yoichi's sole surviving relative (he'd seen to that himself). It was his by right!
Two, he could have his brother back.
One little-known property of quirks is the existence of quirk ghosts. All for One had found that when he took a quirk he would see the ghost of its former wielder in his dreams. Usually they'd just yell at him, but he could often still use them to learn about their quirks. It was likely a similar piece of Yoichi was inside hisquirk. If All for One were to reclaim it…
A fire lit inside All for One. A purpose. He set out, building back his empire, seeking his enemy with all his resources.
His brother's quirk had refused to be stolen. He needed to find a way around that. He found a geneticist who'd been thrown out of academic circles for his (brilliant) theories on quirks. All for One recruited him and gave him everything he needed for his research. Together they surmised that the only thing that could break his brother's will was an even stronger will, born from sheer hatred.
That would be easy. The ponytailed man had taken everything from All for One. He hated him enough.
Eventually he defeated his enemy in battle and tried to take it. But again it failed. No matter how hard he pulled or how much he focused his hatred of the man who'd taken his brother away, the quirk wouldn't budge. Why did Yoichi have to be so stubborn?
The carrier of his brother's quirk laughed. "Didn't you know? One for All can't be taken. Only given." Unfortunately before All for One could…"persuade" him, the man expired. The light in him died.
All for One did everything he could to revive his sworn foe, even trying to resurrect his body. But nothing worked. The light was gone. His brother was gone again.
Five years passed with no sign of Yoichi's quirk. No new bearer, no great resistance leader, nothing.
All for One's enthusiasm faded. The malaise returned. All for One's empire once more began to decay. He kept it running out of sheer inertia. With every day even getting up felt heavier. With every day All for One felt more bored with life. He began to think he'd never get out of this.
Then a hero named Lariat appeared, carrying Yoichi's quirk. All for One laughed maniacally as he did battle with the hero, trading barbs with a glee he had not felt years. Lariat escaped by the skin of his teeth, but All for One performed some wicked schemes that soon had him back on his tail. In the meantime, All for One worked on a solution to Yoichi's stubbornness
To take Yoichi back required a hatred All for One could not conjure. He'd become too drained over the years. He needed someone else's hatred. Yet how could he pass on his quirk without losing it or he trust someone else with his quirk?
The good Doctor felt that with enough time and study he could find a way to artificially replicate All for One's quirk. He also theorized that if All for One's quirk had a piece of All for One's will within that he could use that to control the user. At last! He finally had a way to get his brother back!
All for One knew what to do. He couldn't let One for All go into hiding again. His brother's vessels wanted to be the hero, so he became the big bad villain. The evil mastermind plotting the downfall of the nation, mass murder, world domination etc. The bad guy that the heroes had to chase. He tried to capture the users so he could torture the quirk out of them, but when that failed he'd always look the other way so they could pass it on. He could wait until he and the doctor had a properly hateful vessel.
All for One would take Yoichi back. He would lock him away within his mind. Then his brother would finally be safe. Finally be his. Then the story of how he became the greatest demon lord could truly begin, with Yoichi as its reader…