Warning! MAJOR unmarked spoilers ahead!
- In the series' opening scene Jae-hoon sets a rat loose in a snake's cage and watches as the snake hunts it down.
- Jae-hoon cuts open a rabbit, poisons his father's fish, drowns his father's dog, and murders his father — all in one episode.
- The first on-screen murder. A girl asks a woman for help because her dad was injured in a car crash. The woman goes to help, then gets attacked and killed in front of the girl.
- A family on holiday find their campsite is all closed up. They decide to spend the night there anyway. When the mother goes to fetch water she gets caught by the serial killer. When she escapes and runs back to the van she finds her husband has disappeared. So she tells her children to lock the doors and hide, then sees the serial killer go up to the van and break the window. She runs out and draws his attention. He kills her then goes back to the van.
- Her older son, Mu-won, gets his little brother Mu-chi to hide in a suitcase. The killer notices the suitcase and tries to open it. Mu-won runs out and tries to escape. The killer catches him, drags him back in, and almost kills him.
- One of Jae-hoon's victims is burned to death. We get a nightmarish glimpse of his burnt corpse with smoke still rising from it.
- Daniel Lee goes to an amusement park late at night to meet someone. He gets into one of the Ferris wheel's cars and very quickly gets killed when the serial killer shows up. We don't see his death, just his blood splashing on the window.
- The end of episode four gives the impression Ba-reum is a kidnapper and serial killer. On top of how horrifying that possibility is, it means there are possibly three serial killers on the loose at the same time.
- Mu-won is murdered on live TV while his brother is Forced to Watch. When the police find his body they see the killer wrote "I am the Almighty" on the wall behind him.
- Bong-yi is attacked in a church. She manages to fight off the killer long enough for the police to arrive, and when she's in hospital she remembers he was right-handed. Ba-reum is shocked, because the killer who attacked him was left-handed — confirming there are at least two killers.
- The increasing hints that Ba-reum is really Jae-hoon. Confirmed in episode eight when he sees a mouse and has a flashback to the aforementioned "feeding a mouse to a snake" incident. Then unconfirmed in episode nine when it's revealed he isn't Jae-hoon, but Seo-joon performed a Brain Transplant and gave him Yo-han's brain. Then reconfirmed in episode fourteen when Ba-reum discovers that not only is he a serial killer, but Yo-han was innocent.
- Seo-joon cheerfully admits to taking his victims' heads to experiment on.Seo-joon: I even cut open the heads while they were still alive.
- Ba-reum learns he has a "switch" in his brain that will turn him into a psychopath when it's triggered. And when it's triggered at the end of episode eleven he implicitly kills his cats and possibly kills a child too. Mercifully the next episode shows the child is alive and well... but two of the cats aren't.
- Ba-reum finally figures out who the real killer is.Ba-reum: It was me. It wasn't Sung Yo-han. It was me.
- Seo-joon cheerfully admits he harvested Yo-han's brain while Yo-han was still alive.
