Okay, so I think I’ve got a solid timeline of events for villain!Katsuki
Katsuki leaves UA just before the provisional license exam, he’s at the point where he no longer things the hero system can be fixed without outside intervention. He’s done with how the media acts, he’s done with the blame being shifted to victims based on their quirks, and he’s done with the lies he’s been fed all his life.
Prior to this he started researching and planning what actual change would look like and require.
He meets up with Dabi, who’d only let the rest of the league know about the bet a few days prior. He was curious if Katsuki would keep his word or not. Dabi prods Katsuki gently, saying he knew the kid took honestly seriously but he’s still kinda surprised to see him go this far over a bet. Katsuki replies explaining it’s more than the bet. He talks about what he’s seen, and the failure of hero society to change. It’s more than enough to convince Dabi.
Dabi brings him to a secure location, then relays information to Shigaraki. Shigaraki is hesitant, he’s surprised Katsuki came back to them, but when Katsuki explains his side of things, and with Dabi vouching for him, Shigaraki decides to let him in. He can understand what it’s like to fall for hero society. He did for awhile too. Katsuki just needed more time on his own to realize that.
There’s an introduction period, the league is still figuring themselves out and most of them are hesitant to trust Katsuki. It’s about a week before he’s properly reintroduced to everyone and gets the sense that they’re not headed in any particular direction.
After that week, Katsuki decides “fuck that, I’m going to do shit, y'all are coming along” and since Shigaraki doesn’t really have anything better in mind, he decides to go along with Katsuki’s plans.
UA started panicking as soon as they realized Katsuki was gone, but nobody really knows what happened to him yet. Since he took all his stuff with him, all signs point to him having run away.
Izuku refuses to believe that, and most of the class holds out hope for awhile, but as no evidence turns up, more of them start to think it might be true and start discovering the amount of harassment and guilt he was facing.
There’s about a month and a half period in which Katsuki focuses on getting the league in shape. During this period, they do a forcible take over of the Shie Hassaikai.
Overhaul is killed, Eri goes under Katsuki’s care, and the league is established as a much more powerful force.
The Hero Commission is starting to get nervous, but not overly so. Not yet.
After this month and a half, Katsuki makes his first public appearance as a villain.
He’s in an updated version of his costume, most of it has stayed the same, but he’s added what looks like muzzle to the costume that covers his mouth. The muzzle actually functions as face shield/oxygen system so he can move at much faster speeds safely.
Katsuki’s villain debut is a full scale attack by the league on the Hero Commission directly. An event meant that the Hero Commission had gathered in a relatively accessible location, and the villains take advantage.
Importantly, Katsuki is enforcing a policy of limiting needless harm or deaths, because it ruins their message. The league reluctantly agrees, so they’re being a bit more careful than they were before. However, they’ve still got a hit list for this party.
The list consists of officials who they have confirmation deliberately manipulated polices or actions of heroes at the cost of people’s lives, all for some private gain. They do a lot of damage, and manage to kill 7 people on their list.
Katsuki’s first kill happens here. Previously, he’d managed to defeat Overhaul, but wasn’t quite able to kill him. He just wasn’t ready.
Shigaraki took care of it for him, and told him it was fine if he needed time to learn to kill. He was kind and understanding about it in a way that was genuinely weird to Katsuki. Not only because this was all about murder, but it was the first time in his life that he failed and an adult supported him rather than punishing him for his mistake.
At this event, one of the people he takes down tries to appeal to him by offering him a position as a hero to save themselves, and that pushes him over the line. He ends it quickly.
The league escape afterwards, and footage plus eyewitnesses confirm Katsuki was there, and he didn’t say a word to anyone he attacked.
UA, the Hero Commission, and the media collectively assume / sell the story of him having been brainwashed. It’s the only thing that doesn’t make them look horrible.
Aizawa has been weighed down by a lot of guilt during all of this. He should’ve done more, should’ve reached out, but he’d been waiting for Katsuki to come to him. He’s tried to find Katsuki, but Katsuki has purposefully avoided facing him in battle as Aizawa is one of the few people he doesn’t feel like he could hurt or allow one of the other villains to hurt.
The attacks start getting frequent over the course of another month, primarily targeting the people behind the scenes of the heroic’s system, with a lot of other targets of corruption along the way.
On his third attack, Katsuki has his first run in with Izuku, the first of any of his classmates to face him in battle.
Katsuki refuses to speak a word to him, despite the fact Izuku heard him giving directions to Toga, and the two clash.
Katsuki manages to pull ahead in their fight, but he’s distracted from their fight by someone caught in the crossfire. He diverts course to protect them. Izuku was so hyperfocused on Katsuki he didn’t notice the person. It throws him off enough that Katsuki is able to win their fight complete his goal.
After their clash, Izuku’s finally realized Katsuki isn’t brainwashed. Given what happened, he can only assume it really is Kacchan making all these choices, which leads him to reflect on why.
Between slowly uncovering what Katsuki was suffering through, and watching Katsuki’s actions and their very real impact, Izuku finds himself struggling with the idea of heroes as well. Most of Class 1-A and society as a whole really are.
Shouto & Izuku talk, and Izuku confirms that it really is Katsuki. Not a brainwashed version of him.
At the end of their conversation, Shouto assumes that because it’s really Katsuki, they’re gonna go join him. He defects, and is a bit surprised when Izuku doesn’t follow him right away.
Dabi almost has a heart attack over this, but he can’t really judge.
Shouto is accepted into the league after he and Katsuki talk and he apologizes for not having done anything. Katsuki doesn’t blame him for it, as he understands Shouto had no real sense of what was okay.
Eri gains a new older brother, and she could not be more delighted.
During this period, after the first attack, Hawks was sent in to infiltrate the league. Katsuki can smell the commission on him from a mile away, but tells Dabi to kind of let Hawks in anyways.
The league begins slowly working Hawks out of the Commissions control, before finally after about three months, Hawks realizes how shitty they are and defects properly.
A big part of this happens after Hawks finally comes to terms with the fact that Katsuki isn’t brainwashed, and after Hawks meets Eri and realizes how happy and safe she is with her new family vs how he felt at that age with the Commission.
By the end of that three months, a number of other class 1-A kids and a few kids from 1-B have dropped out of the hero course, or in some cases, UA entirely. The public at large has started to become more disconnected from the hero system as they start to see some of it’s major flaws. The Commission comes under more and more questioning and is seriously losing power.
During this same time, you’d expect to see an increase in crime, but you actually see the opposite.
Katsuki has been very careful and forged an alliance with the MLA such that they’ve been able to crack down on certain types of crime (domestic violence, quirk kidnappings, sexual assaults, etc) while also steadily funneling money into getting social services in theses areas.
This means that you start to see less crime, people feeling safer and more secure, even as the hero commission and system is crumbling.
All of this comes together after a year or two of solid in the form of the government submitting to a major reform driven by figure heads planted by the MLA, but only after the league manages to eliminate the last key figures standing in their way.
Since so many heroes have either fallen from grace, stepped back from the system, or been killed in certain cases, Izuku ended up as an unwilling symbol of peace due to his connection to Katsuki.
Izuku is tasked by what’s left of the heroics system with stopping Katsuki from killing the final major figure whose all that’s standing in the way of the reform.
Izuku, in the end, makes the active choice to step aside, giving Katsuki the key he was given to the room so Katsuki can get to the person to kill them.
Izuku finally decided that he’s had enough of this too, and he’s done defending a broken system based on ingrained ideals that don’t add up.
Aizawa is watching inside the room up in the rafters, he’s stayed a hero of sorts but still functions like he did before.
Inside the room, Aizawa had the chance to cancel Katsuki’s quirk and stop him from killing the person. Instead, he chose to close his eyes and let Katsuki go through with it.
Katsuki looks up to where Aizawa is once it’s done, and Aizawa realizes he knew he was there the entire time. He hops down out into the open and speaks plainly as he always did.
“I’m sorry.” Is all he can really say. There’s so much he’s sorry for. For not speaking up. For letting Katsuki be put through so much. For letting him be driven to this.
Katsuki looks at him for a long, long moment, before he finally looks away and shrugs his shoulders. For the very first time in years, he speaks to a hero. To the only hero who ever tried for him, even if it wasn’t enough.
“S'okay. The problem was bigger than you every could’ve fixed.”
“I should’ve tried. I should’ve done more.”
Again, Katsuki needs a moment to consider that.
There’s silence for a few moments, and then Katsuki’s radio crackles to life. Dabi’s calling him back.
They share one more glance, and Katsuki turns on his heel and walks out.
Aizawa watches him go. There’s nothing else for him to do. His right to change this story ended when he failed to speak up all those years ago.
