Responses
Ok so I've never read this but it has a serial killer
https://www.mangago.me/recommend-manga/interview_with_a_murderer/
If it's not that you can look through the tag if you want
https://www.mangago.me/tag/serial%20killer/

i remember only little details but, there was this serial killer who always got away with his killings. the killer had a relationship with the mc but i don’t really remember the dynamic. there was a third person in this relationship where the killer helped? this third guy kill his past lover and from then on this guy became obsessed with the killer to the point of imitating him and replacing him. big drama big everything in the mix with the three of them