It’s unfortunate because this is such an important theme, but Shigaraki isn’t exactly the most effective messenger for it. Shigaraki barely interacted with hero society while he was in All for One’s care until his villain debut. And by that point, of course society would reject someone whose modus operandi appeared to be destruction for the sake of self-satisfaction. Society didn’t reject Tenko as much as they refused to protect him in his moment of need.
It was Kotaro and the Shimura Household who rejected Tenko, and if anything, Kotaro was the one expressly rejected by hero society. Kotaro, like Kota and Twice, was an orphan because of the endless conflict between heroes and villains. Society abandoned him to his fate and left Kotaro without the space to more productively air his grievances with heroes/hero society. He took out that rejection on his own family and Tenko/Shigaraki internalized that rejection from Kotaro.
But Shigaraki must represent this theme because he is Izuku’s villainous foil. Before obtaining One for All, Izuku was rejected by hero society because of his quirklessness and his embodiment of heroic ideals without a quirk. Izuku was given an impossible opportunity to achieve acceptance, but his foil must be someone who stews in those feelings of rejection and lashes out at society. It’s notable that no one responds to refute this speech from Shigaraki, but the paneling makes it clear that Izuku is listening. He can’t forgive him, but Izuku was attempting to understand here.