The relationship could start with hatred. Bakugou may initially be enraged that Deku has become a villain. But as time passes, he starts to see the reasons behind Deku's transformation, which could lead to a more complex relationship.
One concept could be that Villain Deku manages to manipulate Bakugou into joining his side. Maybe he plays on Bakugou's insecurities or his desire for power. Another could be a world where they are both villains from the start and they form a power - hungry duo, causing chaos everywhere. A third concept might be that Villain Deku kidnaps Bakugou as part of a grand scheme to get back at the hero society that they both feel has wronged them in some way.
It could start with Deku taunting Bakugou. As a villain, Deku would have a new - found confidence and would use it to get under Bakugou's skin. He might leave cryptic messages for Bakugou, hinting at their past and how Bakugou used to bully him. This would make Bakugou angry but also curious about what Deku has become.
Well, AO3 is definitely a great place to start. It has a tagging system that makes it easy to find the specific type of fanfic you're looking for. When you search for 'villain deku x bakugou', you can also filter the results by things like rating, word count, and completion status. Fanfiction.net also has a decent selection. However, you need to be a bit more careful there as the quality control isn't as strict as on AO3. You could also look into Tumblr. There are a lot of My Hero Academia fans on Tumblr who might post about or recommend good villain deku x bakugou fanfics. Some of them might even have their own fanfic blogs where they share their work.
A possible plot is a power struggle between them. Since Deku has turned villain, he might see Bakugou as a threat to his plans. Bakugou, on the other hand, wants to bring Deku back to the light. This creates a constant back - and - forth of battles, both physical and psychological, as they try to outwit each other.
One possible motive could be a long - held grudge that has festered over time. Maybe Bakugou's constant bullying in their past finally pushed Deku over the edge. Another could be some sort of mind - control or dark influence that made Deku act out of character.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.
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.
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.