I was thinking that it was the police chief, mostly because of how he kept on getting in the way and because he was in charge of the case way back. Most recent episode... and I no longer think it's him. They revealed him too easily.
Maybe it's because of how they baited us with the cult leader and the executioner before, or maybe it's because I'm also watching 365 (utterly unpredictable, new theory every episode), but I AM NOT satisfied with it being the Cheif. I think they're just trying to create a cliffhanger, we'll find out next episode.
There's 5 more episodes to go so it's too early for a reveal.
Also super worried for Dong Baek! Hope baby's well!