Just like the title. Perfect example is Strong girl nam soon (Ryu Shi Oh) and i know Vincenzo with villain that falls for female lead. So similar kdrama. Its okay if the story is bad, i mean, i watch strong girl nam soon and survived because of him. 

You even watched Vincenzo and survived XD congrats.

For female leads as villains:
The K2 - this one does not have a good storyline. Actually there are so darn many faults its at times annoying. But the chemistry between the female villain and the ml was amazing.

If you dont mind more greyish leads id actually recommend you to watch Misty. This actually fits also what you look for but not completely since its not side characters. Still Misty is a gem :)

For male leads as villain:
My Name - Its a little tricky with "falling" in love. They never truly have romance scenes like that but there is however very very strong chemistry between them.
Money flower - the best revenge kdrama ever produced but it is very heavy in storyline. He is not per say however a villain, more a very grey character....or maybe a puppet master would fit it better.

And one of the most extreme cases:
Somebody - this is arthouse so be aware of extreme scenes. if you are ok with that however I do think it is a very very well done psychological drama. The male lead is a psychopath and a killer, and obsessed with the fl.

Maybe Bloody Heart. He was quite compelling.