Currently, even more so than All For One, Tomura Shigaraki is the number one public threat to the Hero Society, which is now an uncontrolled fury in search of Dekus One For All. But the antagonist of My hero academyis actually just one of the many puppets of the symbol of fear.
Looking back at the work, numerous details can be guessed at that previously went unnoticed. So one of the fan theories more disturbing and darker than the story of Kohei Horikoshi.
In Chapter 235, titled Tenko Shimura: Origin, we discover them Past of Shigaraki. He was apparently born without a Quirk into a family where the word hero was taboo. Actually the Little Tenko's father was the son of Nana Shimura, who abandoned him to protect him from All For One's goals. Tenko suffered from a strange allergy that caused him an annoying itch, a real ailment that only subsided with the manifestation of decay. But what if in reality, just like Yuga Aoyama, the Decay wasn't Tenko's original quirk and this allergy was a way of rejecting Oneness?
According to one theory It was All For One who donated the Quirk Decay to Tenko. At the beginning of Chapter 235, we see a mysterious man bringing Shimura's baby home. In reality, this man seems to have been the very symbol of fear, and in that moment he gave the little one that frightening uniqueness. Tomura Shigaraki is one of All For One's victims.
That's very plausible. By their own account, All For One loved controlling the heroes' most problematic situations. As grandsons of One For All Vestiges, Tenko was targeted by the Symbol of Fear. Bribing the child and making him his heir was a devious plan to sully the legacy of Nana Shimura and her student All Might. To support this hypothesis, we recently saw that Dabi was one of All For One's puppets in case the Tomura Shigaraki plan didn't go through.