I haven't come across one, but you could try looking on specialized comic websites or social media groups focused on comics. Maybe someone there knows if such a comic exists.
A 'villain deku x villain todoroki fanfiction' might focus on their backstories that led them to become villains. It could be that they were both wronged by the hero system in some way. Deku might have been rejected too many times or faced unfair treatment, and Todoroki could have some family - related issues that pushed him to the dark side. The story could then show how they form an alliance and start their villainous journey together.
The reactions of pro heroes to Deku fanfiction could vary greatly. Pro heroes who are more outgoing and friendly, like Mirio, might actively seek out Deku fanfiction just for fun. He could be the type to laugh out loud at the creative and sometimes wacky ideas in the fanfiction. However, more serious pro heroes, such as Aizawa, might initially be skeptical about the whole concept of fanfiction. But if the fanfiction has some deep insights into Deku's character and his journey, Aizawa might start to respect it more.
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.
Their relationship could start with mutual self - interest. They both recognize the power and chaos the other can bring. For example, Toga's ability to transform could help Villain Deku infiltrate places, and he can offer her protection in return.
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 possible plot could be that Villain Deku is on a mission to undermine the hero society and crosses paths with Hero Deku. They engage in a fierce battle where Villain Deku taunts Hero Deku about his ideals. In the end, Hero Deku manages to get through to a small part of Villain Deku's conscience, leaving him with some doubts about his villainous path.
One great 'Best villain Deku fanfic' is 'Villainous Deku: The Dark Ascent'. It shows Deku turning to the dark side in a very believable way. The author does an excellent job of portraying his inner turmoil and the reasons behind his transformation. Another one could be 'Deku's Villainous Reign' which has some really intense action scenes where villain Deku battles against his former friends. It's a thrilling read.