Hiya, I skimmed through my lists and found some recommendations.
- I Reincarnated as the Little Sister of a Death Game's Murder Mastermind and Failed -
A manga. Fits your interests well. The art has a lot of character and conveys subtle emotions beautifully. I liked the story A LOT.
- Classmate -
A manwha. Checks all your boxes too.
- Captive Marriage with a Psycho -
A manga. Probably the closest to what you are looking for. Unfortunately there are only very few chapters atm.
- The Male Lead is a Murderer -
A manwha. I would have liked for the story to take a different turn but alas. It was still a good read.
- hotaru no yomeiri -
A manga. ML is a killer through and through but he would go to the ends of the earth for FL. Prepare tissues
- Courting Death With A Robber -
A manwha. I don't remember much but I remember that I liked it.
- Sekai o Sukutta Saikyou Yuusha ni Stalker Sareru Mura Musume no Hanashi -
A manga. Different vibe though. ML is not a serial killer (more of a stalker), but he has the same creepiness of a serial killer. Unfortunately only a couple of chapters atm.
- I Thought It Was a Fantasy Romance, but It's a Horror Story -
A manwha. It's honestly not what you are looking for but you might still like it. Everyone's sus, especially ML, and a lot of people die. But it is too funny! It's a Rofan turned Eldritch horror. FL was so dense she changed the genre lmao it's a fantastic read when you need something to lift your spirits

Hear me out: I NEED MOARRRR black-flag, irredeemable ML who are toxically obsessed/ "in love" with FL. None of that he had his reasons or misunderstanding tropes, give me a psychopath who kills and is insane to everyone yet is somewhat nice to one random girl. A series like this, but more romance lol. Stockholm syndrome, twisted love, whatever.