Well, it could be because some fans see a complex and often tense relationship between Deku and Bakugou in the original story. In the series, Bakugou has bullied Deku in the past. So, some fans might explore this darker side of their relationship in fanfiction, imagining that Deku could hold deep - seated hatred for Bakugou due to all the past abuse.
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.
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.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
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.
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.
Bakugou's character might develop by becoming more empathetic. At first, he may be his usual brash self, but as he sees Deku in distress, he starts to understand the importance of being kind. So he grows in his ability to show compassion.
People like these fanfictions because it shows a different dynamic between Bakugou and Deku. Usually, they have a more competitive or combative relationship in the original work, so seeing Bakugou be caring is a fresh and interesting change.
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.