Deku just wants Shigaraki to at least respect instead of destroying. That's why he does everything he can to try to "get into his mind" (now literally) and convince him of that.
Shigaraki's power is one of the worst due to destruction. He himself knows this and has always lived limited in himself due to the terrible education he received from his family (who really didn't know what to do with his power and feared the worst, which actually happened to them) and the hypocrisy of the world in which they live. (not unlike our own world). All For One just allowed him to have an "adolescence" (being kind of his "father") and for him to be able to reason what he wants for the future, in this case, to make maximum use of his power, given that people feared this power.
Shigaraki winning, the world of "MHA" ends and the villains win, even though they lose.
Deku managing to change Shigaraki's consciousness, even with the current losses, the heroes win and the world takes a breather... It's "almost impossible".
That's why I say that Deku is more left-wing than a lot of left-wing people out there. Deku is almost utopian in his quest to be a hero who saves everyone and tries to create a better world for everyone, even at the cost of his own life.
(one thing that crossed my mind is that a character that could be similar to Shigaraki but from another manga/anime is Fuuko from "Undead Unluck". She was lucky that Shigaraki wasn't. )