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.
At first, it could be extremely strained. Deku might be terrified of Bakugou and avoid him at all costs. But as the story progresses, if Bakugou realizes his mistake, he could start trying to make it up to Deku. Deku, on the other hand, might be hesitant to accept Bakugou's apologies at first, but over time, they could start to rebuild a more positive relationship based on mutual respect.
One possible theme could be bullying and its extreme consequences. Bakugou's statement is an extreme form of bullying, and the fanfic might explore how such words can deeply affect Deku's mental state. Another theme could be redemption. Maybe the story will show Bakugou realizing the gravity of his words and trying to make amends. It could also touch on the theme of friendship, as Deku might have to decide whether to forgive Bakugou and rebuild their relationship.
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.
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.
The relationship could start with hatred. Bakugou may initially be enraged that Deku has become a villain. But as time passes, he starts to see the reasons behind Deku's transformation, which could lead to a more complex relationship.
Growth is also a prevalent theme. Both characters are on a journey of self - discovery and improvement, and fanfics usually show how they grow both as individuals and in relation to each other. For example, Bakugou learning to control his temper and Deku becoming more confident.