Bakugou might be initially shocked. He's used to Izuku calling him 'kacchan' for so long that it's become a normal part of their relationship. So, when Izuku stops, it could throw him off balance.
One possible reason could be that Izuku has grown up and wants to have a more formal relationship with Bakugou. As they progress in their hero training and development, Izuku might feel that the childhood nickname 'kacchan' no longer suits their new, more mature dynamic.
I'm not sure specifically as there could be many different takes on it. It might be a fanfiction where the character Izuku reaches a point of apathy or stops caring about things he previously held dear, like his dreams of becoming a hero or his relationships. It could be due to some traumatic event or a slow build - up of disappointments in the story's universe.
All Might would probably be one of the first to try to stop Bakugou. As Izuku's mentor and a symbol of justice in the story, he wouldn't tolerate such violence. Another could be their teacher, Aizawa. He's strict and protective of his students, so he'd surely step in.
In this kind of fanfiction, it usually shows a build - up of tension between Bakugou and Izuku. Bakugou might have been constantly pushing Izuku's buttons with his aggressive behavior and taunting. This could lead to Izuku finally reaching his breaking point. For example, Bakugou's over - the - top insults about Izuku's abilities or his dreams might be too much for Izuku to bear. And when Izuku snaps, it could lead to a big confrontation between them, which can be a very dramatic and interesting plot point in the fanfiction.
If Izuku stops holding back in fanfiction, he might achieve his goals much more quickly. He could show his true power and skills without restraint. For example, in battle scenarios, he could defeat his opponents with ease and surprise everyone around him.
A great way to develop their relationship is through shared traumas. Maybe they both had a difficult experience during a villain attack. They could open up to each other about their fears and insecurities during that time. This vulnerability would bring them closer together. Additionally, they could participate in a hero competition together. As they face tough opponents, they encourage each other and celebrate their victories, which deepens their bond.
One possible reason could be that Izuku realizes Bakugou's unchanging and hostile attitude. Despite his efforts to be friends or get along, if Bakugou continuously rejects him or is overly aggressive, Izuku might feel it's a lost cause.
In this kind of fanfiction, a likely consequence is that it changes the whole dynamic of the story. Their classmates might take sides. Some might side with Izuku out of sympathy, while others might try to understand Bakugou's actions. It could also lead to changes in the school's view on Bakugou, perhaps implementing stricter rules or counseling to prevent such incidents in the future.