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.
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.
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.
I recommend the book "Master Li's Trajectory". The main character, Li En, had transmigrated to the infinite world and experienced the challenges of fate such as his sister being captured, his father being seriously injured, and his mother being knocked unconscious. At the same time, he was also the son of a Pharaoh, a professional cheat, and the successor of the Eight Leaves Wife-Breaching Sect. He believed that the Onmyoji in this novel was somewhat similar to the Yin-Yang Master. I hope you like this fairy's recommendation. Muah ~😗
Uraraka would be heartbroken. She was one of Deku's closest friends. She might lose some of her usual cheerfulness and become more focused on finding out who or what was responsible for his death. She could also struggle with her own self - confidence in using her quirk, as Deku was always an inspiration to her.
One possible plot for this fanfiction could be that Deku is facing a powerful and unknown enemy that has the ability to nullify his powers. He's in a desperate situation when Kurogiri shows up. Kurogiri, for reasons unknown to Deku at first, uses his portal - creating abilities to rescue Deku. It could turn out that Kurogiri has a connection to Deku's past that he's not aware of. Maybe Kurogiri was once influenced by someone who saw the same kind of potential in Deku that he himself had when he was younger, and that's why he saved him.