One common theme is friendship. Since they are classmates in the My Hero Academia universe, many fanfics explore how their relationship grows from just schoolmates to really good friends. Another theme is mutual growth. For example, Bakugou's hot - headedness can be tempered by Kirishima's more level - headed nature, and Kirishima can learn to be more assertive from Bakugou. Teamwork is also a frequent theme as they often work together in the story, whether it's in training or in actual hero missions.
I'm not sure if there is such a specific fanfic available. Fanfic topics can be quite diverse, but this one seems rather unethical in the context of the characters' relationships. You could try searching on major fanfic platforms like Archive of Our Own or FanFiction.net to see if it exists.
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.
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.
Well, in these fanfics, their relationship can be seen as a sort of mutual growth. Kirishima might inspire Bakugou to be more open and less abrasive. For example, Bakugou could learn from Kirishima's ability to make friends easily. Meanwhile, Bakugou's drive for success can push Kirishima to improve his own skills. It's like they bring out the best in each other in the fan - created stories.
One popular theme is their teamwork. Fans love to write about how they work together in battles, like combining Bakugou's powerful explosions with Kirishima's hardening ability to defeat enemies. Another theme is their friendship growth. It shows how they start off with a somewhat rocky relationship and gradually become closer comrades.
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.