Since I think the drama won't revealed what truly happened regarding GJC case (serial killing case as very well what truly happened on the snow scene with HB & HE as well) until their very later ep.

I think I'll write down my theories here just to check them later whether it turns out to be true or not ㅋㅋㅋ

So here's my theory:

My take was, when little GJC was abused by his mom, GJC sort of create an alter ego/ other personality (beast) who loves to kill, while the original GJC remain as an innocent lil' kid & this was the GJC we're seeing in the drama.

When GJC was prosecuted for the 11th victim case, we were shown that GJC got a seizure whenever he suffered an extreme situation and his lawyer plead that he won't remember what happened after that, thus later he was sent to mental hospital because he can't be punished for something that he can't remember.

So, from original GJC perspective the 11th victim is his first victim (because the other 10 victim were done by his alter ego & he was "unconscious").
The beast (portrayed with GIW face) keep persuading him to kill, but GJC refused by saying he's different as unlike him, he can feel love.

That day, when he accidentally killed HB, his alter ego did the following multiple stabbing to HB.
Right after that, HE cursed him & GJC shot himself in the head.

BUT, I think he never actually shot himself, but it's morelikely just a methapore in his mind that he had killed/thrown his original self and have been taking over completely by his alter ego (the beast).

That's why he survived the shot & sentenced in jail. 

This theory is consistent with what HB said in the interview room, according to him GIW had been pronounced dead 15th years ago, never married & doesn't have a son.

Also, when GJC visit his father at that abandoned school, we see him stabbing his father & his father's painting, but later we see his father was totally fine & the painting still in a good condition when HB found it.

So, continuing the snow scene at HB&HE wedding day, I think it's possible that GJC, who was already taken over completely by his alterego, was the one who smashed HB skull & performed some ritual so that he can be reborned as HB.
(I think he hide & burried HB's body elsewhere, put him in formaline, etc so HE didn't find HB's body & was looking for him later in snow just like what being shown on SH's dream).

Because I think there's no other person who's as obsessed as GJC with this reborn concept aside of  GJC himself. 

But luckily, earlier at church HE had tied her fate with HB & later she cursed GJC back too, so I think GJC wish to be HB was ungranted as it has been countered by HE.

I'd like to add another thing, regarding the scene where GIW pick GJC with a car & said that he was about to leave & then ask Hye Mi to look after GJC.

I think it's GJC alter ego himself who was about to leave because of GJC determination to love HE, and his alter ego ask Hye Mi to keep monitoring him.

However, GJC way to collect himself was ruined after finding out HE need a heart transplant to live thus he killed the Salon's owner.

I had the same theory before I watched the latest episodes and preview of episode 13 (25/26). Considering the trauma and ridicule he suffered after being stabbed in one eye by his step-mother, it wouldn't be a surprise if Ji Chul created an alter ego, his father, in order to protect his mind and to justify killing women over his rage of abandonment. And I think it's the logical explanation with all that's been going thus far. However, I think the writer going with the route "he's actually innnocent" and ignoring everything that makes sense in order to romanticize a mentally sick character. The way Ji Chul stalking Ha Eun and trying to give her his heart without her knowledge or consent isn't romantic, it's creepy, disturbing and plain sick. So I really hope they don't go with "killer daddy is real" route. 

Ikr?! Unfortunately they're doing it again in order to bring JI Chul character into another light no matter how it'll turn the story becoming more absurd, like:

Why even Hye Mi had any interest in making HB reborn as GJC & made him burried like that? It doesn't make any sense. 

Why even LTH want to help JB & wait for a whole year & provide him with anything just to make him taken his revenge while he's not even sure if JB will be able to make it out alive?

GIW is still alive? He must be at least 80 this year, will he even still had the power? Lol

Why overcomplicated things when they can make it simple if they proceed with GJC alterego route & it will answer every possible question without having to make a dozen weird people doing numerous nonsense action & wasting too much screentime for all those side characters that we never care about? Lmao

 Dreamy:
Why even Hye Mi had any interest in making HB reborn as GJC & made him burried like that? It doesn't make any sense. 

I don't think Hye Mi wanted Hyung Bin to be reborn as Ji Chul? She clearly said she wanted the detective to be reborn like Ji Chul, which is absurd because how would she know if Gong Ji Chul would be reborn? Not to mention the whole tying his hands with the book is so superstitious, it's not even in Hye Mi's character to do so.

 Dreamy:
Why even LTH want to help JB & wait for a whole year & provide him with anything just to make him taken his revenge while he's not even sure if JB will be able to make it out alive?

Because it's more makjang that way, they don't even care if the plot sounds so ridiculous.

 Dreamy:
GIW is still alive? He must be at least 80 this year, will he even still had the power? Lol

Well, I still believe Ji Chul did all the killings, while the recent ones are committed by Sang Ah. Gong In Woo, if he really exists, probably drew inspirations from the murder to draw. Like Hyung Bin said, one killed, the other drew.

 Dreamy:
Why overcomplicated things when they can make it simple if they proceed with GJC alterego route & it will answer every possible question without having to make a dozen weird people doing numerous nonsense action & wasting too much screentime for all those side characters that we never care about? Lmao

They probably don't even have enough material for 16 episodes, they're really stretching the story for as long as they could, not even bothering answering some real questions. And even with the current timeline, I feel like the characters are just wasting time instead of moving the plot along.