Definitely not. Deku's character arc in the manga is centered around his growth and development as a hero. There are challenges and difficulties, but he never turns to the dark side.
Deku remains a hero in the manga. His determination and sense of justice keep him on the path of righteousness. There are no indications or plot developments suggesting he becomes a villain.
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.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
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.