Sometimes I have the idea that in the final fight with Deku, Shigaraki was fighting in and out of control with himself vs AFO, and he knew he wasn't winning, he probably knew the others weren't going to either.
So, when he was himself, he thought that if this boy could be a good person, he would get him the fuel he needed to keep fighting. Thought that maybe, he could help make him a good hero.
A good hero, and a chance at a good society.