I haven't finished any dramas where the FL's main love interest is a serial killer, but I have finished a few dramas that are in the ballpark (e.g., where the FL's main love interest murders one person, is a murder suspect, or is the brother or son of a serial killer or where a secondary male lead with romantic interest in the FL is a serial killer):
- Healer - The ML becomes a murder suspect at one point.
- The Smile Has Left Your Eyes - The ML is a murder suspect for a while. Later on, he does actually murder someone after being provoked.
- I Hear Your Voice - The ML tries to murder someone (sort of out of self defence) but the FL gets in his way. He is later suspected of killing the same person.
- Misty - The FL and ML are both murder suspects. A secondary male character with romantic interest in the FL kills multiple people, driven, in each case, by a desire to get revenge for the FL or to protect the FL.
- W - At different points in time the ML and FL are each suspects (the ML for murder and the FL for stabbing the ML) and they are both connected to the villain who is sort of a serial killer.
- Come and Hug Me - The ML's father is a serial killer.
- HIStory3: Trapped - One ML is investigating the other for his connection to a murder. It's a Taiwanese BL.
- Hello Monster - The ML is suspected of being involved in murders and is connected to two serial killers.
- Secret - The FL's initial boyfriend (the SML) commits a deadly hit-and-run, and the FL takes the fall for him.
- Pinocchio - The ML's brother commits murder... iirc it was a revenge killing for their parents' wrongful death.
