Who do you think is the killer?

@Sujach Well, I think that the story took it's own life?  Don't think there is anyone behind it. It either changes when Yeon Joo goes into W world and in those moments she is kind of leading the story, or when Kang Chul takes his life in his own hands (like with the car accident part) then he is leading it. We could say the most conscious person is the one who creates it at the moment. The more free will the characters gain the more story changes coz it's starting to be based on their own decision and not what Sung Moo draws. 

Just a hunch, somehow, when Sung Moo lost his will to continue the story because of his family issues, some supernatural thing happened. Why? Because he didn't reveal the truth in the murder of the family, yet his plan was to kill the main character. It isn't logical for a writer. It was also unfair for Kang Chul as a character, that's why, he stole the writer's will and it resurrected his own life. Now, Kang Chul, maybe has the control of the whole manwha. Of course, Sung Moo is still the creator, there's a way to counter Kang Chul in the future, I guess.

@Brenda: Well, if Kang Cheol really has the control of the current manwha, maybe, him, deciding to kill himself can be a way for Sung Moo regain the power back on W. He has to re-write again and finish what was undone. But. who. of. us. will. want. that. ?? haha! Anyway, that's why Yung Joo came to place, she will be the catalyst, I think. :D

Based on episode 3 everything that happened before the bridge scene was the intended work, without any alteration from " anything ", of the writer ' Oh Sung Moo ', so the actual killer is just a character that only ' Oh Sung Moo ' knows about & will only know, unless he actually shares the information about his ' inspiration ' for that character, so the REAL killer is ' Oh Sung Moo ' but the actual killing was done by his character.

The character dressed in black that appeared during episode 3 as well as on the roof top of the building, before ' Oh Yeon Joo ' met & saved ' Kang Chul ' is PROBABLY not the killer that killed ' Kang Chul ' family but another creation that ' Oh Sung Moo ' made, solely focusing on killing ' Kang Chul '.

Also off topic but during episode 3, i think ' Kang Chul ' already knew that ' Oh Yeon Joo ' was invincible, that why he was able to shoot her to try to get information out of her, because she was PROBABLY shot before & nothing happened to her, when that character dressed in black came in with the gun & might of accidentally shot & hit her but didn't do anything to her, hence why there was an exaggerated scene when ' Kang Chul ' was looking at her while fending off ' that ' character.
Or he finally realises he has some sort of power in his world to make things turn out the way he wants, something like that, who know hahaha

@Martin: You got it! The killer of the family remains to be a mystery because the motive wasn't clear and Sung Moo didn't elaborate more., only it was shown that he had the gun and wearing black. The masked man wearing black who is targeting Kang Chul however is created by Sung Moo. If they were indeed one person, then the killer is the mastermind and it will be the creator, of course.  That's where Sung Moo's power hold, I guess he can control the creation of characters in the webtoon.

Okay... now that I'm reading all these theories I have a major feeling that Kang Chul himself is behind it. THIS IS ALL SO CRAZY AND I LOVE IT!
Maybe he's real, he's the reference. And he wants to end it since W became so popular and it's all about his life. Maybe the Kang Chul from the real world found a way to go in as well and is trying to eliminate his "character" self who's becoming real while living a life he never got the chance to because he was ruined after being released from jail because, let's be honest, what are the odds of a murder suspect climbing all his way to the top and becoming Korea's most favorite person? I find that social loop to be quite strange... 

Does it matter who the killer is inside the comic?   The author of the comic is the ultimate culprit.  He planned it out and committed it in cold blood with his computer screen and his stylus.   So if Kang Cheol ever gets into the "real" world,  the target for his vengeance will be pretty certain.

I agree with you Svetlana.
I have a feeling that the mysterious killer is KC himself. Because now he is in the real world he might be able to enter back into the webtoon world at certain intervals. But he now wants to exist in the real world he had to get to where he is in the webtoon world. This meant that his family had to die etc. If none of those existed then it means he can exist either thus meaning that he might have to become the murderer for him to exist in the real world. You need to have a cause for the effect. It would seem like a vicious cycle. I might be completely wrong here but it's just food for thought.