so I didn't expect this manhwa would actually discuss serious stuff like mental illness.
First thing you need to know is that the ML isn't MC's brother, but the step brother(Elliot) of her brother(William).
And basically MC's mother got postpartum depression when she had MC, and she was wrongfully treated by removing her uterus that made situation worse (because it caused her to be in pain constantly) that in turn caused her to want to kill MC time to time. And the dad was killed when he tried to protect MC from her mother
It's probably a genetic thing too seeing both MC and her brother also had a similar kind of illness.
And yes, the ML isn't a psychopath locking up women, it was just the MC's illness acting up. That locked room was actually William's room.
It was really touching in the end when the MC decided to face her past and her delusional disorder, and went to search for the right treatment, which was a difficult thing to do considering the time the story was set in.
She also tried to leave ML to avoid torturing him like her mother did to his father, but the ML was supportive and stuck with her til the end, which hits me real hard in the guts (;д;)
Went in for incest and left with a deep touching story discussing about mental illness treatment. Whaddya know, there's actually a gem hidden right here behind that fake incest setting.