Episode 3 has a big clue that points to who is the Master that the kidnapper serves. 

####Don't read further if you don't want to know####

In ep 3, On the shooting date between the parents, you see Saeon mother look at an article about the bombing on her phone.

In the very next scene, the Master texts the article to the kidnapper while he was cutting up fish. Obviously, the kidnapper is her real biological son who likes to eat fish  and she controls him.

I agree with you that she might be more involved than we think, but I think that would be too easy. My money is on her child still being alive and they're working hand in hand.