As the final battle between heroes and villains rages on in My Hero Academia, Tomura Shigaraki stands as the most dangerous village alive. Not only does he pose a direct threat to the heroes attempting to stop him, but more importantly he presents an existential threat to all of humanity.
Shigaraki was raised to lead the destruction of My Hero Academia's world in more ways than one. The grandson of Nana Shimura, the seventh holder of the One for All Quirk, he suffered abuse as a child from his father for his love of superheroes. His father hated superheroes due to the fact that his mother Nana gave him up for adoption when she was not able to care for him properly while being a superhero. His hatred for humanity developed after being punished by his father, triggering his "decay" quirk to power up. Unable to control it, he killed his entire family. While he did not mean to kill his sister, mother, and grandparents, he actively attacked his father. With no home, Tomura wandered the streets until he was found and taken in by All For One. Sensing Shigaraki's immense hatred for humanity, and heroes, under All For One's analysis, Shigaraki presented the ultimate tool finally get his revenge against All Might and reclaim the One For All Quirk. From his point forward, All For One treated Shigaraki as if he were his own son.
Despite Shigaraki's already immense powers, All For One, and his assistant Dr. Kyudai Garaki, believed that in order to truly be effective, he needed more than his Quirk offered. Subsequently, via surgery, he was given enhanced abilities including most notably, the original One for All power. With the combination of his pre-surgery abilities which included the ability to disintegrate anything he touches, with his post-surgery upgrades, Shigaraki becomes My Hero Academia's most powerful villain and easily surpassed the capabilities of his mentor All For One. Now, he poses the most significant threat to the authority of the Pro-Heroes since All For One in his prime. However, with all the quirk effects that were passed down from his grandmother, along with those he inherited after being given the All For One Quirk, he's increasingly proving that the once ridiculed doomsday theory of the Quirk Singularity is true.
According to My Hero Academia's Quirk Singularity theory, Quirks are combined and passed from parents to children. As the combination of Quirks become more complex with each successive generation, there will come a time when the resulting Quirks will become too complex and powerful to be controlled by the holders. That is, when the world is filled with people with Quirks of unimaginable power, that cannot be controlled, humanity will suffer. Indeed, a Quirk will eventually arise that has the power to eliminate humankind altogether.
Indeed, there is no better example of the Quirk Singularity's existence in My Hero Academia's world than Shigaraki. Shigaraki embodies the future of humanity in the present. First, he's imbued with a potent mix of Quirks that are, at present more powerful than ever encountered before. Second, there is a question of whether Shigaraki is able to control his abilities. This is most obviously illustrated in the tug-of-war battle he is currently undergoing with the vestiges of All For One in My Hero Academia, as well as the grotesque deformity of his left arm that displays all the hands of the people whose Quirks have been assimilated by One For All. As dark as it might seem, Shigaraki might just be the greatest evidence for My Hero Academia's quirk singularity theory.