quesosoup:
"The apple doesn't fall too far from the tree."

Now that you mentioned this, I thought of something. We were shown in the earlier episodes that Do Do Hee's parents were killed in an accident. However, we are not given many details about the accident. It also seemed like Madam Ju felt guilty about it. Noh Seok Min also mentioned in one of the last episodes something about how dare Madam Ju judge him. Let's also add that the scar guy seems afraid of someone coming after him. 

I do find your theory about the scar guy being Seok Min's dad plausible. We are never shown how or when he died. I was also thinking it could be Do Do Hee's dad, but then why would he try to kill her?

 I don't know how all of this is related or if it's even related, but it seems, at least to me, that we have been getting small clues about who it is. 


Noh Seok Min: Most likely not the killer. 

Noh Do Kyung: Psycho, but it would be too obvious. However, I do feel like the killer is connected to him in some way. 

Joo Seok Hoon: Again, it would be too obvious to make him the killer. Instead, I think he plays a big role in Do Do Hee and Jung Koo Won's past.

Jin Ga Young: Most definitely connected to their past life and probably one of the direct or indirect reasons he became a demon in the first place

Kim Se Ra: She knows something. 

I find your theory of some of the family members being related to Guwon's past life really interesting! Steming off of that, a fleeting thought kind of came to mind. What if someone in the family made a contract with Guwon, and what if that person was Madam Ju? I call this thought fleeting because I can't really make much sense of it and a lot of clues point to it not being plausible, however, I think it'd be interesting if the true cause of her death was a contract she made with Guwon, and perhaps she killed herself before Guwon could send her to hell after the contract were to be over. 

I do agree that if Dokyung were to be the killer it'd be too obvious since he's the very first suspect, that's why I'm leaning more towards it being Seokmin, if it is anyone in the family who directly killed her.  Although, it being Seokmin is also quite obvious, which is why I think there has to be way more to it.

I also never thought of/noticed how Madam Ju seemed to have felt guilty regarding Dohee's situation.  The reason for it could be because her parents were probably the victims of Seokmin's drunk driving incident. Like you mentioned though, Seokmin's reaction to Madam Ju's scolding made it seem as if she was being hypocritical, so maybe she has something more to do with it? It makes more sense if he was referring to something about her own past though.

Everyone in the family for sure knows something that Dohee doesn't know about Madam Ju's death though, and I'm really curious as to what it is!