"It's not possible to reason with Shigaraki"
This is a point I have often found myself refuting during my many (and sometimes painful) debates with MHA readers. When I talk about a peaceful way to solve the conflict with Shigaraki, by talking to him and offering him help, I am often told that talking to him would not work, that it's impossible. I think a clarification is necessary : Shigaraki has never been closed to dialogue and the heroes have never tried any other way than repression by violence. Let's look at the facts now.
The USJ attack. Yes, he is the one who attacks the heroes but when All Might arrives, Tomura criticizes him for the brutality of the heroes, a criticism that All Might rejects without listening. Obviously, he doesn't have to listen to the criminal who comes to kill teenagers, I am well aware of that, but it's one of many examples that show that heroes have never tried anything other way than to respond with violence.
The shopping mall where Shigaraki came to meet Deku to express his frustration and exchange with him on what differentiates him from Stain. Even when Deku irritates him by telling that Stain doesn't run away from difficulties unlike him and that he says that Tomura is not fighting for any ideal, the latter remains relatively calm, listens to what Deku tells him and learns from it.
After kidnapping Bakugo, he tries to put him at ease and make him understand that society despises and rejects those who differ from the norm, in order to propose him to join the League (which is literally a refuge for rejected people with nowhere to go and who want to destroy the system that oppresses them). Bakugo doesn't listen to him of course, it's hard to argue with the one who just kidnapped you and put your comrades in danger, but while Shigaraki talks, we can see that his point is relevant. Look at this journalist who publicly insinuates that Bakugo's aggressive temper would make him a potential super-villain.
And I answer right away to those who say that it was an attempt of manipulation from Tomura : Not at all. It would be totally out of character, Tomura having nothing of a manipulator. On the contrary, it's him who is victim of them and who must be saved by the protagonist. We have also this panel which shows us that he was quite serious when he spoke to Bakugo.
This scene where as Twice gets hysterical and Toga threatens him with a knife to the throat after Tomura tells them they're going to associate with those who killed Big Sis Magne, he chooses to calmly explain his decision and show the trust he has in them by revealing his face, instead of getting nervous and trying to kill Toga like at their first meeting.
Same here, Spinner gets legitimately angry when he sees that they have no solutions to improve the society and gets upset against Shigaraki who tried to explain himself without violence but is interrupted by the arrival of Machia.
And here we have the best example. After almost getting killed in his sleep by the "heroes", Shigaraki is lectured by Endeavor about his presumed lack of ideals (lol, that's so ironic), to which Shigaraki (whose scorched body is cracking and still regenerating) responds with a speech about how criminals like him are only the consequence of the intentional neglect of heroes and civilians, that this is a systemic problem and that he's now fighting to destroy this toxic system that always rejects him. A powerful speech that Endeavor doesn't care about and answers with another murder attempt.
It will be pointed out to me that in some situations, Shigaraki attacks first to try to discuss later. Yes indeed, but when you have been traumatized as a child several times, brainwashed since the age of five by a sadistic psychopath who also exploited your traumas to make you an emotionally unstable weapon of mass destruction and cut you off from the outside world for more than a decade, it's rather logical that you are not very good at establishing a healthy discussion. However, it can be noted that from the arc on Overhaul, his social skills have become much better, as evidenced by his interactions with the League.
It's not possible to say that one cannot reason Shigaraki because we can see here that talking with him is possible, but also because heroes never tried. Even when All Might found out the truth about Tomura's identity and he wanted to find him to help him, he was quickly dissuaded by Gran Torino who told him not to see Shigaraki as anything other than a villain (when he is mostly a victim, but okay...) and that this kid, who is just the descendant of his dead best friend for whom AM and him were responsible, is only a bastard beyond all redemption. So please stop pretending otherwise and try to consider that violence is not the most effective solution to a conflict, either in the short or in the long term. Because even if the heroes kill Shigaraki, it won't change the status quo. Through its negligence, the system will create new victims who will rebel again and again against the source of their repression.