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.
Midoriya's friends like Uraraka and Iida would be extremely worried. They would immediately start looking for him. Uraraka might use her quirk to search in the air, while Iida would lead the ground search with his speed. Bakugo, on the other hand, would be furious. He would probably go on a solo mission to find Deku and take on Todoroki without hesitation.
The characters would have a range of reactions. Some would be in disbelief. They have witnessed Deku's journey from a quirkless boy to a powerful hero - in - training. So, seeing him as a villain in a fanfiction would be like a complete 180 - degree turn. Midoriya's mother would be devastated. She has always supported her son, and to see him as a villain would go against everything she believed in. The teachers at U.A. would be concerned and start to analyze what could have led to this change. Maybe they would question their own teaching methods or if there were any signs they missed during Deku's time at the school.
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.
It's hard to say for sure. Deku has grown a lot, but All For One is a powerful villain. It depends on various factors like their current abilities and the circumstances of the battle.