SPOILERS FOR ANIME ONLY FANS
All For One revealed something to All Might during their fight at Kamino Ward. He figured out Midorya Izuku was the hero’s successor. But, did the Mastermind behind the Villain Alliance reveal that same information to Shigaraki?
From what we currently know about AFO, he has no reason keeping this revelation to himself, unless, he wants his pupil to figure it out himself. But, how can Shigaraki figure that out if he doesn’t know about All Might’s quirk? You can’t. AFO keeping it a secret makes no sense with our current knowledge of the situation. Now, assuming AFO told his student about Deku, what can Shigaraki do with this information?
At this point in the story All Might, even without access to One For All, is still a living symbol of peace. True, he’s retired and that in itself creates ramifications being felt now and throughout the rest of the series, but as bad as things have gotten it can get a lot worse. All Might is still the heart of the hero community, still the symbol, and Shigaraki knows it. All Might is dead, but his spirit lives on in Toshinori. If Shigaraki can kill him, the world will once again change. The forces of evil are stronger without All Might, but the good still out weighs the bad. Toshinori’s death will tip the scales, evil will become the dominant force putting the heroes on the defensive.
After Shigaraki kills All Might he’ll have plenty to time to focus on Midorya. A good plan except for one little problem—Deku himself. The ninth user of One For All is growing stronger, adapting to his power more each day. Deku won’t become All Might; he will become someone more powerful—a new, brighter symbol of peace capable of relighting the hearts and hopes of the people. Deku’s death isn’t required to destroy society, but it’s required to keep it that way. And unfortunately for Shigaraki, he won’t realize this until it’s too late.