Why did AFO (All For One) groom Tomura Shigaraki (Tenko Shimura) when he just a little kid?

AFO saw potential in Tomura's quirk and recognized him as the grandson of his former enemy, Nana Shimura. AFO wanted to use Tomura to seek revenge against the hero society and continue his own legacy. He groomed Tomura to become his successor, molding him into a villain who shared his ideologies, motives, and desires. By doing so, AFO hoped to achieve his ultimate goal of controlling all of Japan.