okay so i forgot which chapter, but when finn was having a mental breakdown and the vision of her former self handing her a rope might be related to the killings. its possible finn tied up her father for noah to kill, because her former self did allude to her having more involvement than she remembered. in the case it was also mentioned that all the victims were tied with a rope. so the new killings might be a reflection of how her father was killed, but instead its finn in her fathers place.
also, there was a flashback where finn was seen making a promise to noah that if she ever felt the will to live, that he should kill her. now that finn has (almost) moved on, he’s now making his way back to her. the killings of the girls who have similar situations as finn are probably the build up to finns death.
it's so beautiful to see heejae realizing that he can't take garam's hate. he's treated every student with the same cruel and strict teachings, but heejae cannot resist the person and sound that garam is. he's now seeing how the hate he initial wanted is coming back to bite him since he's being blocked out from garam's world, and in turn the true depth of his sound that he carves.