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.
For Ruby, seeing the Jojo fanfiction would be like opening a new world of possibilities. She might start thinking about how some of the Jojo concepts could be incorporated into RWBY. Weiss would be more reserved, but if the fanfiction had a really good plot structure and character development, she'd give it more respect. Blake would be intrigued by the different ways the Jojo characters interacted with each other, and might compare it to how the RWBY team bonds. Yang would be like the cheerleader of the group, loving all the explosive action and big personalities in the Jojo fanfiction. In conclusion, their reactions would vary from Ruby's eager acceptance to Weiss's more cautious approach.
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.
Ruby's worldview might expand to include more complex power systems like the stands in Jojo. She could start thinking about different ways to use her own powers. Weiss might become more open - minded about different types of stories and the importance of creativity. Blake could see new ways to approach the idea of identity, as Jojo characters often have unique identities tied to their stands. Yang might realize that there are different levels of intensity in battles and might strive to bring that into her own fights in RWBY.
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.
Bakugo would likely be furious. He'd feel betrayed and would probably be the first to attack Deku, yelling at him about how he's thrown away everything they've worked for. Uraraka might be more empathetic. She'd try to understand what drove Deku to become a villain and hope there's a way to bring him back to the side of good.
Bakugo would be deeply affected. He might initially deny his feelings, but then would be filled with guilt for all the times he was mean to Deku. He could become more withdrawn and focused on becoming a hero worthy of Deku's sacrifice.