In the first episode, the news article shows the heading Son and Father. What if his father (the biological father who was named in the article)was the serial killer and he maybe had to hide the bodies or something?

I don't think the "parents" we met were his real parents, at least not the dad.

His name used to be Do Hyun Su. Now it's Baek Hee Seong. There's a girl, one we can assume was killed, named DO HAE SU-the reporter has her necklace and reminisces on her. The name implies she is our ML's sister. My initial idea, especially since someone tried tying him to something and murdering HIM as well, is that they were going to make him the fall guy for the real Baek Hee Seong's crimes perhaps OR maybe the real Baek Hee Seong killed his sister and he snapped... I think perhaps the comment "we're all in the same boat" by t he "dad" at the birthday dinner was blackmail-instituted ie the dad and his real son, the Baek Hee Seong guy, might be the real killers and this psycho happens to have proof or at least acts like he does...

We know he had to run out of town, and we know he was almost killed. Maybe those "parents" of his tried killing him and he could testify on them at any point/has some sort of evidence against them; maybe he's realized he cannot win the fight and settled for monetary security of a sort; maybe they're all in it together in some twisted way.

I also got the weird idea that maybe Baek Hee Seong and Do Hyun Su were HALF brothers ie the same mom, different dad hence her feeling of possession over him. 

I'm loving it already. Shows like this make me think "yes, tvN and OCN really ARE sister networks, aren't they?!" :)

Do Hae Su is alive as seen in the trailer or the cast list. I did mean the real biological father. 

The theory you wrote about half sons does sound possible but I get a feeling they will do something much more.

I'm just hoping he's not the real serial killer.

All we've seen so far is Hae Su in the past tense (granted the other rambling thoughts were from 1 episode and the second mostly makes me incredibly nervous having seen him in that raincoat and knowing about the man he used to live with who betrayed him in a hundred ways)... I keep thinking he's trying to chase down the real serial killer, perhaps, but they're sure making him a fatal blend of perfect dad/husband/cool artist attractive and utterly terrifying! :)

I'm thinking that his sister will be the unlikely serial killer maybe? and that he just has that disorder where he can't recognize emotions of others but isn't actually involved in any murders, people just assume that he is based off groundless rumors. That at least seems like the direction the show will take, rather than him really being a psychopath, which would've been interesting too.

my opinion is ( sorry if i say any spoiler) he become murder in order to protect someone and because he dont know how to show emotion everyone thinks that he is the murder, i dont know but i bet the psychiatric guy has some relation with all this, and he probably he is kidnapped the journalist because of revenge and because to not relieve the idenity, as for the sister i bet the psychiatric guy did it. i am probably wrong.... ^_^
p.s sorry if i dont write names i have always problem with memmorize them <3

I agree with some of the theories going around. I don’t think those are his real parents, although they are aware of at least part of his past. It seems odd that he would run away, change his name and still be in such close contact with the people who could tie him directly to his past.

While obviously he is a killer in some way, I think the man he killed was doing something to the girl. Like someone above said, he probably did it to protect her but because of his inability to understand or show emotions properly, his actions were mistaken as that of a cold-blooded killer. It probably didn’t help that his dad was suspected of being an actual serial killer either. Along those lines, I think his father was the true serial killer and is most likely still alive. I also don’t think ML/Baek Hee-sung was the one who killed that old roommate dude in the restaurant and I doubt that the lair is his either, since that pool seems reminiscent of the pool he was tied to and left to drown in back in the first episode. The episode probably wanted to implicate him in the murder though by showing him come in from the rain in similar attire.

While I don’t think that the real Baek Hee-sung is out there and also a killer (what a coincidence that our lead runs away from a suspicion of murder and a serial killer dad only to take up the identity of another killer??), I do think that someone from ML’s past, either his dad or someone with intimate knowledge of the murders, is playing games with him and killing anyone who might get too close. I also believe that to some degree ML/BHS cares for his family. Maybe he doesn’t fully understand or feel human emotions to the extent others do, but there is something else at play within him, even if he himself doesn’t know that. Perhaps show will have things happen to his family that have those feelings reveal themselves to him.

He obviously loves his family, really and truly. If you watch the clips from the trailers and how he interacts with his daughter you can see he really cares. My main question at this point is why his "mother" gave him Baek Hee Seong's identity and why Baek Hee Seong is in the father closet. Does the mother even know about it? Whoever the killer is, they have knowledge of details only Huyn Soo and his real father should have.

Honestly, I really don't think Huyn Soo is the real murderer. As some of you all mentioned before I think he is covering up for someone. And he definitely could not be the killer now. The flashback to when he was going to kill the roommate in the forest, He didn't do it cause he remembered his father. He is trying to run away from becoming like his father. Whoever the killer is now will eventually kidnap Huyn Soo and try to kill him. The big questions are Who? and Why?

Though Hyun Soo does show sympathy to his wife and daughter amidst all of his acting, you have to have a severe lack of  emotions to watch the videos on emotions and to even get to the point where you lock someone in your basement for a few days without any guilt. 

I personally think that (or would like it if) the hallucination of his father may be what drives him to kill and he himself doesn't have a large drive to do it unless the figure forces him to. Since his wife keeps his father away, he probably hasn't killed/had the urge to kill since they got married and possibly only killed out of necessity like he had tried with the reporter. I do believe that the incident where he almost killed in the woods, but didn't is good reason to doubt that he even is a killer. There was also the comment about him that the reporter made about him being crazy but honest so I think that even if Hyun Soo didn't actually kill anyone, he might at least believe that he did.  If not a killer, he at least has a very severe personality disorder.

I really enjoyed the American show Dexter and have been waiting for a similar drama, but so far all of the similar Korean dramas have chickened out of making their main characters full-fledged killers (if they really are, then there's usually a female cop chasing them who is more of a main character) so I wouldn't at all be surprised if he wasn't actually a serial killer. As much as I would like my theory to be true, idk if Korea is ready  for a genuine serial killer protagonist, though this show seems to have the most promise for this being the case out of all similar K-dramas so far.

So far ep.6...

I have no idea who is the partner killer, but that should be man, not woman, and I'm really hoping that it is not the ML. 

Maybe his sister killed that head of the village...  But who killed that taxi's wife...

Looking forward for the next week!