That hasn't been revealed yet. We just know Heeseo's mom and Taekyung's mom were both involved in a murder. And Taekyung's mom killed Heeseo's mom before she could confess.
My theory is, Heeseo's mom lead the man to Taekyung's mom who killed him. She probably found Taekyung's mom's rosary in the man's had and hid it as evidence. I believe it was also Taekyung's mom who buried the man but the other guy who recorded it thought it was Heeseo's mom bc he saw her bringing the man into the forest. Heeseo's mom probably felt guilty for leading the man to his death and also why she chose to flee and hide bc she knew Taekyung's mom would not mind killing her and Heeseo(which she was spot on about) if she ever found them to eliminate the evidence. She probably also kept paying the blackmail money in hopes the guy will keep his track shut about her whereabouts and that she Heeseo would be safe.
Can someone remind me what happen to his mom I don’t wanna reread lol