Dark Thriller With Psycho ML

• Iyashi no Otonari-san ni wa Himitsu ga aru 
• I Love My President Though He's A Psycho 
• Good Bye, My Princess 
• Story of Kunning Palace 

Not quite but similar - these have very unhinged/toxic MLs:

• Broken the Heart 
• Circle of Love 
• Enslaved by Love  
• Long Summer Wei Yang 
• Rising Feather 

And maybe...

•  Bulgasal: Immortal Souls (possibly? it's not romantic at first but he's definitely obsessed to a very unhealthy degree)

Somebody - well he is a psychopath and a killer and obsessed with her. She is also kinda on the verge of madness. It is really one of the most interesting dramas for the specific degree you look for. It is however arthouse and thus not the liking of everyone nor is it something for the faint hearted ;) (hence the rating)

Also if you enjoyed Somebody you may want to watch the movie "The uncle". This is on the same degree as somebody. A disabled girl, who is abused by her care takers, reencounters her uncle, who is the convicted murderer of her mother. But nothing is as it seems. It is very very disturbing (arthouse genre) but truly interesting.

There is also a British show called "The killing kind" , that also focuses on a psychopathic, obsessed male lead and a woman who tries to escape him. It is quite interesting.

Then there are some dramas that fit your wish to some degree:
World of the Married - amazing drama but literally everyone is absolutely crazy and psychopathic.
Misty - this one follows a woman who focuses on her career but ends up being involved in a murder case and needs now her husbands help to get out of it. It is an amazing drama and has a theme that you look for however I cannot truly explain that well since it would spoil everything.
Maestra: Strings of truth - this one is also a bit diverse. The male lead is mad in his own way but has kind of a good heart, the other male lead is crazy and the female lead is also kind of a unique person. You will probably enjoy the male lead a lot ;)

Also, if you are interested in webtoons Id recommend you "I Failed to Oust the Villain". It is completed, is a sort of reincarnation story where the female lead tries to get rid of the psychopathic male lead, who is obsessed with her. It is a really good, unique webtoon.