Well, 'Villain's Unlikely Love' is a great piece of fanfiction in this genre. In it, the villain has always been in the shadows, hating the hero. But as the story progresses, through various encounters and shared moments, the villain begins to see the hero in a different light. It shows how love can cross the boundaries of good and evil. The way the author portrays the gradual change in the villain's attitude towards the hero is really engaging.
There is also a fanfiction called 'When the Villain's Heart Melts'. Here, the heroine has a unique quality that the villain can't resist. It might be her intelligence or her ability to see good in everyone. As he gets to know her more, his feelings change from hatred (or indifference) to love. The story then follows how they deal with the fact that he's a villain and she's the heroine, which often goes against the norms of their world.
A frequent theme is the power of love to change. The love the villain has for the hero can make them question their entire belief system. Also, there's the theme of secrecy. Since they are from opposing sides, they often have to keep their relationship a secret, which adds a lot of tension. In some fanfictions, this secrecy is what drives the plot forward, as they try to hide their feelings from their respective allies.
It's the contrast. The villain is usually associated with bad things, and when they fall in love with the heroine, it's a complete change. It's like seeing two opposite poles attract.
A good example is 'Twilight'. Although Edward could be seen as having some villain - like qualities at first, he falls in love with Bella, the heroine. The story is filled with their romantic and sometimes tumultuous relationship against the backdrop of a supernatural world.
There were many such animes such as " Your Name "," Evil Crown "," Even a chuunibyou needs to fall in love!" " About My Reincarnation as a Slime "," Sword God's Domain "," Gate of the Fate Stone ", and so on.
Often, it starts with the villain being intrigued by the heroine's defiance. For example, if the villain is used to everyone fearing him, the heroine's lack of fear catches his attention. This initial spark of interest can then grow into something more.
Overall, when the villain falls in love in the manhwa, it often leads to unexpected plot twists and complex character growth. The villain might struggle with their newfound feelings and face challenges from their past.