So... Shigaraki and Izuku used to be friends when they were kids.

While Shigaraki was in the park, Izuku had found him sitting alone on the swings. izuku, decided to approach Shigaraki and become friends with him. When Shigaraki showed his quirk to Izuku, thinking that Izuku would leave him, Izuku instead thought that it was an amazing quirk and soon began to ramble about how it would be such a great way to save people.

However, when Izuku was in the park trying to find Shigaraki, he saw nothing but a note attached to the swing they always played on. It was a goodbye letter from Shigaraki and Izuku was upset that Shigaraki had left.

Of course, it wasn't Shigaraki's intention to leave him but he had to because that's what 'sensei' told him to do.

Soon enough, Izuku had forgotten about Shigaraki but Shigaraki didn't. He kept a picture of Izuku because he was his first ever friend that didn't judge him nor was scared of him because of his quirk being dangerous that people started to call him a villain.

Timeskipping to the present, Shigaraki and his team attacked UA once again, specifically Class 1A. Izuku, accidentally went too far ahead and lost sight of his classmates and teacher. As Izuku finally got out of the cave, Dabi and Toga had found him and made a plan. they told the plan to Shigaraki and he agreed.. Even if he was against seeing his childhood friend die and turned into a Nomu, he still wanted to have Izuku by his side so he agreed to it even though he was upset and angry at himself for letting Izuku die.