taeyudong

You're right. And I think the question can be extended to how could the killer figure out about blackouts when Yebun was not even using her power to investigate. The killer maybe is not a psychic but he still looks like he knows about it because of the aspect of the murders. Maybe at the beginning he didn't have blackouts intentionnaly; it could match the split personality theorie that someone guessed above (which for me is the most likely theory now).

Ooh everything would make sense with this theory !

Yeah it's a good theorie. But what about all the things related to the psychics ? Maybe it would be too much if the killer is a psychic and also possessed. Also there is the fact that the killer at the beginning wanted the others to think that he had closed eyes, because he knew all the psychic things. Shouldn't he be conscious to think about these kinds of details? Why would any spirit pay attention to this kind of things? Aaaah I want to know

I would say the killer is the shaman, Seonwoo found it and was going to tell Yebun. I don't know how but Seonwoo must have found the knife after Jangyeol pursued the killer, and understood that it was the shaman. Then he hid the knife to show and tell Yebun because she was the only person he trusted. That would be why the shaman was looking for the knife in the house (apart from the police request) and why he wouldn't let Yebun and Seonwoo alone in the workshop (not because he was worrying); he somehow suspected Seonwoo was going to tell Yebun.

Or maybe the shaman is really not aware and don't know himself that is he the killer because he is possessed by some kind of spirit.

Also I saw someone in the previous comments assume that maybe the blackouts were because of sunglasses, and I think that would match well if the shaman is really the killer.