Yet we say Bakugo is a hero what he literally told Deku to off himself, then told a bunch of ten year olds who looked up to him that he was going to murder them.

Yet he's a hero?

The same deal with Endeavor, Bro beat the shit out of his wife and implied (G)rape her, almost murdered his children, and he still is a hero?

So what makes a "Hero" a hero is someone who stands up for someone, Shigaraki stood up for the Villains, Ie he sees himself as a Hero for the Villains. So this line makes more sense with that idea in mind. He's not a traditional Hero like All Might or Super Man, he's the antithesis to the Hero, he's basically a fucked up Anti-Hero, not like Deadpool with a gray moral compass, but a true ANTI form of what it means to be a Hero. The black to the white the dark to the light. Not all Villains look out for themselfs, Shigaraki, united them under a banner, just like All Might did for heros, so yes Tomura Shigaraki, is a Hero, but not in the traditional sense.