So, from today's episode, I counted the people who said at some point in episode 9 something related to "stop being a joke" or something similar, and the ones who said it so far have been:
- Dong Baek's mom
- Dong Baek's ex aka Kang Jong Ryul
- The annoying landlord's wife, Hong Ja Young
- Dong Baek herself
And they are all connected to Hyang Mi in some way, so I definitely think the one who's going to die is Hyang Mi, especially with the way she keeps blackmailing people, borrowing money without returning it and having an affair with rich men.
I thought about the possibility of Hyang Mi being the killer too, but from the latest scene of episode 9, when Joker (or at least we assume it's him) entered Camellia, Dong Baek greeted him/her like a customer. And the way Dong Baek even told Hyang Mi to go live with her without abandoning her, I think Hyang Mi wouldn't have a reason to want to kill Dong Baek.