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.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Izuku would probably be terrified at first. After all, he's been kidnapped by villains. But then, his hero - like determination would kick in. He might start looking for ways to escape or at least resist their plans.
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.
In deku harem fanfic, a common characteristic is the expansion of Deku's relationships. It may include his female or male counterparts depending on the nature of the fanfic. The stories usually involve how Deku's heroic deeds and kind heart attract others to him. There is often a build - up of emotional bonds between Deku and the members of his harem. The fanfics might also play with the idea of how Deku balances his relationships while still focusing on his goal of becoming a great hero. This can lead to some interesting plot twists and character developments within the story.
In fanfictions, Mina Ashido might be in Izuku's harem. Her energetic and fun - loving personality could add a lot of color to the harem. Jirou Kyoka could also be a potential member. Her musical quirk and her somewhat reserved nature could create an interesting contrast with Izuku. And then there's Toru Hagakure. Her invisible quirk and her friendly personality could make her a unique addition to Izuku's group of love interests.
All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.
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.