The mysterious Killer

 potato28:

Also, remember how ko hyeong-su’s son, ko yeong-chang was very eager to indict professor yang. It may be because he know that his father is involved. It cannot be just because he hated yang’s guts and that he hated how yang embarrassed his father. 

yeah I think he might the mastermind behind after this week episode and prof Kim is suspicious 

 potato28:

she is very suspicious. I hope she is not a part of the murder. I think she may be appearing anxious because she hid lee man ho's son

but like every law, she passed helped the other guy in the long run.  I think prof yang might suspicious of her too because of the scene in front of the prison,

Its  Prof. Kim . She seems too nice to the point she is is sus. She is either the murder or the instigator.

Also this is my weird theory but she is one of the actors in the main role section(han joon hwi, kang sol a, prof. yang and her) in cast and credit so if she was just a nice prof then she would be in the support role section thus i feel like there is a lot more to her than her "nice" personality.

I might change my mind after seeing the 14th episode but for now it's prof. Kim