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.
In such a fanfiction, the jealousy aspect can also add a lot of drama. For example, if Deku is oblivious to Kacchan's jealousy, it can create misunderstandings. Kacchan might misinterpret Deku's friendly gestures towards others as something more, and this can cause tension. It's a great way for fanfic writers to delve into the psychological aspects of these two beloved characters and create engaging stories.
One possible motive could be a long - held grudge that has festered over time. Maybe Bakugou's constant bullying in their past finally pushed Deku over the edge. Another could be some sort of mind - control or dark influence that made Deku act out of character.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.
Well, 'Deku and Fem Kacchan: A Different Dynamic' is a great fanfiction. In this one, the author delves deep into the characters' personalities. Fem Kacchan is shown as having a more complex relationship with Deku. It's not just about rivalry but also about mutual respect. Then there's 'Fem Kacchan's Redemption with Deku' which is about Fem Kacchan trying to make up for her past behavior towards Deku and how they grow closer as a result. And 'Deku's Support for Fem Kacchan' shows Deku being there for Fem Kacchan during her tough times, which is really heartwarming.
Some characters might try to understand Deku's motives. Kirishima, who was close to Bakugou, would be angry at first but might also try to find out if there were any extenuating circumstances. However, many would be afraid of Deku, seeing him as a threat now that he has killed one of their own.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
Bakugou may prove his love by being protective. He might put himself in danger to save Deku. In many stories, this kind of self - sacrifice is a strong indication of love. Also, he could start to understand Deku's dreams and support him in achieving them, which is a deeper form of love than just friendship.