* spoiler * WHO IS THE CURRENT SERIAL KILLER?

The son of Headhunter Han SeoJun and wife Sung JiEun is Sung YoHan, who is the 10yo JaeHoon in 2005. Yohan takes on his mom's lastname Sung. The woman who stopped JaeHoon from burying alive his stepbrother JaeMin is headhunter's wife. So that confirms JaeHoon is headhunter's son whose IQ is 160 and aimed to be a doctor.

YoHan is my #1 suspect in that being a neurosurgeon, he knows very well the anatomy of the body.  Also, Jaehoon was very obssessed with getting answers from the Church. The current killer's MO (modus operandi) is finger pointing to the church/cross or anything related to cross. He goes about the killing with a submissive accomplice or more. 

He probably didn't kill Jaemin who now lives as Koo Dong Goo  and may likely be the accomplice in crime.

Jung BaeReum is the son of the other lady whose husband, fellow colleague of Daniel Lee, died while saving a kid from accident. She took the gamble that her baby is of the 1%.

 AnastasiaWun:

The son of Headhunter Han SeoJun and wife Sung JiEun is Sung YoHan, who is the 10yo JaeHoon in 2005. Yohan takes on his mom's lastname Sung. The woman who stopped JaeHoon from burying alive his stepbrother JaeMin is headhunter's wife. So that confirms JaeHoon is headhunter's son whose IQ is 160 and aimed to be a doctor.

YoHan is my #1 suspect in that being a neurosurgeon, he knows very well the anatomy of the body.  Also, Jaehoon was very obssessed with getting answers from the Church. The current killer's MO (modus operandi) is finger pointing to the church/cross or anything related to cross. He goes about the killing with a submissive accomplice or more. 

He probably didn't kill Jaemin who now lives as Na Chi Kook and may likely be the accomplice in crime.

Jung BaeReum is the son of the other lady whose husband, fellow colleague of Daniel Lee, died while saving a kid from accident. She took the gamble that her baby is of the 1%.

I saw a Twitter post that said that Ba Reum seems to also know Anatomy based on the bird & grandma..and how the bird got a broken leg(which that user states that in an accident, bird won’t break their legs?) it feels like the user is telling me Ba Reum purposefully broke the leg without saying out loud..and it’s for the purpose of the first meet up with KMC

Hello, let me join the guessing game! :D
So, just to gather everything in one place:
The past serial killer--who got caught in December of 1995--is Han Seo Jun, whose victims were also Ko Moo Chi (the younger brother) and Ko Moo Won's (the older brother) parents. Ko Moo Chi is now a detective, and Ko Moo Won is now a priest.

Han Seo Jun the Head Hunter is now in prison for life, so he cannot be the current serial killer.
Jae Hoon  is his son, and we don't know for sure his current name, however:

 AnastasiaWun:

Yohan takes on his mom's lastname Sung

That would make sense. Sung Yo Han, whose mother is Sung Ji Eun, for some reasons changed his name from Jae Hoon to Sung Yo Han. We saw Jae Hoon in 2005 being at church and talking to God about being different from others, and praying that he doesn't end up being a killer (but he ends up as one anyway).

But, except for Sung Ji Eun, there were also two other women who were pregnant at that time: the woman who talked to Sung Ji Eun and  who chose to believe that her baby didn't belong to the 99% but to the 1%, and the president's wife whose baby also got tested and probaly also got a 99% chance of being a psychopath.
Since Han Seo Jun was arrested in December 1995, I believe that Jae Hoon, the president's child, and the woman's child were born in 1996, with Jae Hoon being born some time at the beginning of 1996, the woman's lady 3-4 months later, and the president's baby being born the last.

(Also, at this point of the drama, based on the information we've been given, we can assume that Jung Ba Reum is one of those three children who were born in 1996.)

Then, we have a scene (probably takes place in 2000) where the kids are at the zoo. We see a kid in a yellow raincoat catching a rat on his way to school, then giving it to the snake later at the zoo. He enjoys the show of the mouse fighting for its life. However, we don't know if this kid is Jae Hoon, or if it's the woman's child, or if it's the president's kid (he would've been a former president at his point, right? If so, the kid would've probably attended a normal school and had no bodyguards with him).

In 2005, Jae Hoon kills a rabbit, kills Dad's fish, kills THE FREAKING DOGGO, and tries to bury his step brother alive. But is there any proof that he ctually killed his family?

In 2020, the current timeline, we can see that the serial killer has a photo of the yellow raincoat kid in his room.

As for now, my picks are rather obvious: Jung Ba Reum or Sung Yo Han, but the latter would be too obvious as he's a doctor, and Jae Hoon wanted to become one. I just can't believe the writer would go for a such obvious killer.
The priest doesn't really seem like he would be able to be a killer as I believe he's paralysed? Although I would love it if he turned out to be the murderer; a priest who in fact hates God. Maybe he became sick and twisted after having witnessed his parents' murder, and almost having been murdered himself.

Also, there's another fucked up man who attacked Oh Bong Yi in the past, and who seems to be released from prison soon. But he's too old to be the current serial killer (and he's still in prison while the killings take place).

From sources like CJen, we know that BR will change due to an accident. It should be the one he got ran over by the car. Then from there he will change but we don’t know if it’s a good change or bad change. It is also stated that he would lost his memories and have the capabilities of understanding the murderer’s intention. That said, if he lost his memories he might be seriously hiding something under that kind smile (making it possible that he indeed could have done some messed up stuff). Then he himself and us will have no idea who he truly is until the very end. As he acts as the ‘good guy’ who lost his memories while trying to find the killer.

this is before CJen re-edited the character bio

 thefoooook:

1. Jang Ba Reum's house looks similar to Jaehoon's and the yellow kid

https://twitter.com/lsgxjs/status/1368233917472407552

2. Edited voice of Jang Ba Reum and it sounds similar to the killer.

https://twitter.com/wdldlsdn/status/1368420414087565312

3. Blood on the pants 

https://twitter.com/leedwsg/status/1368131573682298885

4. The button on the shirt before and after the magic show

https://twitter.com/Igemwoeyo/status/1368215961971482628

5. The killer's not in a good condition

https://twitter.com/itchykdrama/status/1367878014659223554

Omg they are all super suspicious tysm i wouldn’t have noticed if it weren’t for you 

 thefoooook:

I think it's not a mistake but it's a subtle message for the viewers, if you notice, the killer is right-handed but Dr Sung Yo Han holds his bag in his left hand (the last scene of Ep. 2) and he holds his scalpel in the same hand. Ko Moochi is also left-handed. Meanwhile, the killer uses his right hand to stab the woman who ended up in the drain. Jung Ba Reum is right-handed. It's super discreet but the director/writer left us with so many clues. Another clue is Jaehoon and Jung Ba Reum similar taste in checkered clothes.

I hope it's not a mistake, this show is too good to have a mistake like that. But sometimes there are just some errors, for example (not taken from this drama) in one scene a glass is full, but seconds later it's half empty, and then it's full again. Or a character gets hit on the left cheek, but the cheek that's red is the right one.

 thefoooook:

That's why Lee Seunggi said in one of his interviews, "Pay attention to the details because it contains many hidden messages about the characters".

Can I read/watch it somewhere in English? Could you share the link to the interview?

@thefoooook

Thank you so much <3 (I'm on my phone and can't quote rn)