You know, something like Love Between Fairy and Devil. Villain or anti-hero as ML that has romance.

I have a soft spot for villain/anti-hero/bad boys.

Thank you so much for recommendation.

Your Highness - ML becomes a demon king(enemy of all martial world) in a gameworld. It is funny comedy with slight romance.

Mr.Bad  - the ML is a villain from a book, and it just started airing a while ago. 

Love better than immortality - The main lead is the villain

Moon lovers - I wouldn't say the main lead is the villain, but a lot of people do see him that way

The romance of tiger and rose - The main lead tries to kill the female lead at first so maybe that counts?

The  King's Woman

Goodbye My Princess

In both of these ML don't start as an outright villains though they  are  at least a bit shady 

in The Wolf and Rise of the Phoenixes MLs  may not be villains but they def have very dark side to them and both have intense romantic storylines

Technically both of Word of Honor's MLs  can be called villains ( though somewhat retired) in not-spelled-out romantic relationship