Are there any novels where the protagonist is reborn as an antagonist or a villain?

Yes, there are certainly novels that explore the interesting premise of a protagonist being reborn into the role of an antagonist or villain. A prime example of this is "Reborn as the Enemy Prince." In this story, the protagonist finds themselves thrust into the heart of the enemy's territory, facing a war they never wanted to be a part of. This setup immediately creates a high-stakes environment where the character must navigate treacherous political landscapes and potentially conflicting loyalties, all while inhabiting a body that is inherently opposed to their former self or original goals. The inherent drama comes from the internal struggle of being in a position of power that is also intrinsically tied to conflict and opposition.

Another novel that touches upon a similar theme, though with a slightly different flavor, is "Stealing Heroines’ Affections Is My Job as the Villain." While not strictly a rebirth into an antagonist role, it features a protagonist who embraces a villainous path, operating in a world where power is gained through less conventional, even illicit, means. This novel delves into the darker aspects of power dynamics and character motivations, where the protagonist is actively choosing to embody a more antagonistic persona. Both of these novels offer unique perspectives on what it means to be on the 'wrong' side of a conflict or societal perception, and how a protagonist can forge their own path, even when starting from a disadvantageous or morally ambiguous position.