I think the psychopaths we know so far in the series are:

JBR — No doubt that he’s sus. Perhaps he isn’t aware he is one till he encounters the killer as mentioned in the description and his behaviour changes. So many scenes in the second episode seem too unnerving, especially the ending scene where the Doctor and JBR make eye contact and the “ killer ” says all those words. Also I might sound weird when I say this but sometimes I feel like as if his character seems emotionless at some scenes? idk it’s probably just me..

The Doctor — He is definitely a psychopath. I have a suspicion that he is the serial killer but I don’t want to make speculations like that too soon into the series. The killer is someone who knows anatomy very well as Moo Chi states and Doctors know anatomy well. He was also very stern and upfront with the old lady in the hospital then he was later confronted by JBR’s friend.

I think there are at least 3. One being the headhunter, Two being the doctor and I feel like there is someone else who is a secret psychopath that we don't know yet or someone that we haven't met yet

@Abby According to the drama's site, Shin Sang's father is a politician who is running for president.  He is suspected of being a "parachute" which means somebody who got employed through connections.

at the end, when Jung Ba Reum is finally met with his real mother, he says, "I know it's not going to change anything and you may not believe me but I wasn't trying to kill Jae Min (his 'fake' brother)." What was he trying to do then? He was literally burying him alive in the ground. any guesses?