Why did the heroes view Tomura Shigaraki as a 1D-villian after displaying symptoms of unresolved trauma?
I still think about this on occasion and I really think a lot of people forget about it when talking about the villains. There genuinely was no precedent for seeing villains in any context outside of being criminals. Even All Might parrots this idea.
When Tomura was ranting about his need to tear down the symbol of peace at the USJ All Might looks to Midoriya and tells him, to his face, that villains trying to make any kind of argument about the faults of society are deflecting so they can excuse their own desire to do bad. And he warns Izuku not to fall for it, as if it's an intentional deception rather than a crack in the facade that can explain why Tomura is like this and what could be done to help him.
This is why I hate the argument that the ending is weak because hero society doesn't change. I think people making that argument aren't actually clear that THIS WAS THE PROBLEM. It wasn't the billboard charts, it wasn't heroes not being more strictly penalized, it wasn't the prison system, it wasn't the HSPC. The problem at the core of My Hero Academia's society, the problem that let All For One manipulate these isolated and broken people into a threat to the entire world was an inability to recognize villains as human.
This is why the scene with Joki at the end of the manga is so important. Now that the idea that most villains come about from being mistreated or abused with no safety net, people start to rethink how they used to react to these kinds of scenarios. They're seeing suffering first rather than a threat and the fact that Tomura's suffering went on for so long, the fact that things got this bad, will forever be a reminder to never take steps back when faced with people like him again.