When the title of your series is called Monster, the alias of your Big Bad, you might as well ramp the scare factor Up to Eleven. Since said Big Bad is Dissonant Serenity, a lot of the horror is a bit more subtle.
- The OP, "Grain". An effective piece of industrial ambiance, sprinkled with a bit of Ominous Latin Chanting, that sets the tone for the entire story.
- Another scene that scared the crap out was the one where Jan Suk learns his loving bosses are trying to murder him. He decides to do nothing and then they die and then he is blamed for it.
- Most of Grimmer's past is heart-wrenchingly terrifying, but what takes the cake is that the brainwashing treatment that he had experienced as a child left him unable to feel any emotion, even at his own son's death.
- The conclusion of the introductory arc, in the scene where Johan reveals himself to Tenma is absolutely chilling.
- Adolf Junkers had a change of heart after a long, sentimental talk with Tenma. He decided to confess to his crimes. He said he felt like Tenma was like his father. Tenma even bought him the clock he always dreamed of having. And then Johan found him.
- Junkers wasn't the only one victimized in that encounter - Johan made damn sure to eviscerate Tenma's worldview when he said that "every life is precious".
Johan: Doctor... I heard you were promoted to chief of surgery soon after that incident. I'm really happy, Doctor - that you were promoted to that position.
Tenma: What are you trying to tell me?
Johan: You are where you are today because the Director and the others died, right? I'm so glad I was able to help. That was a good thing.
Tenma: What did you... oh my god...
Johan: Isn't that what you wanted, Doctor? That's what you said, anyway, right after I regained consciousness. That's what you told me - that you hated them so much, you wished they were dead... All that I did was grant you your wish, Doctor.
- Roberto and the way he tracks his targets down. Along with tons of Fan Disservice, he's one of the scariest people in the series.
- Nina's backstory, which quite literally drives her to the point of suicide: As a young child, she was abducted, confined in the dark until she lost all track of time, and presented to about forty people who would all shortly thereafter be murdered in front of her eyes because the man who has selected her for his eugenics experiment had had a change of heart.
- Wim, a young boy working at a small town motel, has an alcoholic father who takes all of his earnings. He is also constantly bullied by other kids. And then, after getting beat up in the rain, and having his bicycle stolen, a kind elderly man gives him a gun.
- Everything about Kinderheim 511.
- Johan presenting Tenma with an ultimatum: either Tenma kills him, or Johan kills a little boy.
- The implication of that scene: Johan is about to force the Incorruptible Pure Pureness that is Tenma to do worst crime a human can do. No one is safe from him.
- The fact that Johann is not only a Self-Made Orphan, but a repeat offender at that. After one notable instance, he is spotted by his sister who, unable to quite come to terms with her brother being the reason their foster parents kept dying follows his request to shoot him in the head. They were ten at the time.
- The extremely creepy guy ordered by Johann to make sure that Nina doesn't leave the Heidelberg Castle.
- Martin's backstory - the one night that he decided he was sick of carrying his drunken mother home, she froze to death. The creepy guy who knew Martin's entire backstory and happily explained it only makes it worse. Though that was because Johan told him everything. The shadows on his face and his wide eyes complete with that unnerving smile made it all the more freakier.
- Johann teaching children to play a game that involves falling off of buildings. One of the rules of the game is that, if at first you don't succeed, you try again.
- The death of the Turkish prostitute. It happens off-screen, but you get to hear her dying gasps through a bathroom pipe.
- The bloody aftermath of Grimmer's rage.
- This little snippet from Episode 29: Dr. Gillen is interviewing the murderer Peter Jurgens. He shows Peter a picture of Johann and Peter says that he does not know him, and then he asks him for a pen. He takes the pen and begins shading around the face of the man in the picture. He tells Dr. Gillen that on the second day of the murder, he had gone to the park; he was telling himself that he wanted to stop, and then he saw the man surrounded by children there. Peter then repeats the words "You should come too" over and over again while some ominous little tune plays and the camera does a close up on his expressionless face. He then takes the pen and plunges it into his right ear. Blood squirts out and he falls over dead.
- The "children's books". In particular, there's one where God looked into the mirror to admire the hat somebody had given him and saw the Devil staring back. The face, and then the name of the child...
"Do you know what the god does? I know, I figured it out. Do you know? Do you know what the god did after that?"
- The almost Creepy Monotone way Nina reads the book.
The monster finally had a name,
But all the people who could call him by that name have disappeared.
And Johan was such a wonderful name, too.
- The almost Creepy Monotone way Nina reads the book.
- Nail clippers. Grimmer being tortured by having his nails clipped and broken shorter and shorter, and a few pulled off entirely.
- The whole Ruhenheim arc, when friendly people from a small town start killing each other, with the help of a happy elderly couple.
- Johan's completely blank expression before he has the prostitute killed in chapter 65. Johan's typical, default expression isn't much better. Half-lidded eyes, faint smile and Dissonant Serenity ahoy.
- There's also Johan's expression just after removing his wig and revealing that it was, in fact, him masquerading as his sister.
- Anytime Johan has an expression on his face other than Dissonant Serenity is sure to make the viewer/reader void their bowels.
- "...Well Richard?" "How about a drink?" If you didn't believe Johan was one of the most evil people ever before, that scene should have sealed it for you.
- Right before he asks him if he'd like a drink, Johan realizes that he's got him right where he wants him and makes on of the most terrifying Slasher Smiles ever put onscreen.
- The tape recording in The Scariest Thing. They listen to it further, and then all of a sudden the audio stops. Seconds later, Johan's present voice comes in. Someone somewhere screamed in horror at that scene.
- The Cruelest Thing. Breaking a child by giving him a nihilistic speech about why he's alone.
- Johan telling the boy that his mother doesn't want him goes straight into Fridge Horror once you get to the ending. It turns out that Johan was severely impacted by the idea that he wasn't wanted by his own mother.
- Inspecter Runge goes into a room that Johan had been renting. He had just found definitive proof that Johan Liebert did exist outside of Tenma's head. He goes into the room and...his fingers don't type AT ALL! Creepier than it sounds, as just a few minutes before he mentioned that everyone who can be considered human leaves a trace behind wherever they go but he notes that there was no trace at all in Johan's room.
Runge: I've never before seen a room so devoid of human presence. I am Johan Liebert. Who am I? I was here, but I don't exist. Am I a demon?
- Mr. Hartmann. The way he seems friendly at first, but turns around to Dieter with this angry stare followed by an off-screen beating of the poor kid as soon as Tenma leaves is horrifying. His descriptions of Johan doesn't make things better.
- Neither do all the scars on Dieter's body. The poor kid can't be older than nine.
- Johan Liebert is pure Nightmare Fuel in every scene he's in! Whether it be the way he casually murders anyone who knows anything about him, burns a library down just to see people in chaos, drives mostly sane people to suicide or murdering for him through Mind Rape, that he scares the bejesus out of otherwise hardened criminals and psychopaths, or the fact that he does ALL of this and more with an emotionless smile constantly on his face.
- Johan screaming in terror and passing out in episode 33. The fact that such a monster who until this was calm and emotionless, completely lose it after reading a book, is very distressing.
- During the Lunge vs Roberto fight in Ruhenheim, it was rather nightmarish to see Lunge get the upper hand on Roberto by grabbing his bleeding wound through his shirt and squeezing it.
- Even worse than that is in the anime, you can distinctly see that Runge/Lunge isn't just squeezing the wound - he's deliberately sticking his thumb inside the bullet-hole to aggravate it.
- Another frightening thing about that scene is how Lunge ends up losing control when Roberto simply repeats things Johan told him. Johan's Mind Rape powers are so extreme he can Breaking Speech an emotionless Inspector Javert into a breakdown without even needing to do it himself!
- The last episode seems pretty cheery and optimistic. That is until the end, when we see Johan springing back to life in his bed, reaching out to Tenma and looking as terrified as the people he tormented throughout the series.. Thankfully, it turns out to be a dream...Wait. Where did he go?
- Our Destination Calls, Johan's leitmotif. When this music kicks in, any character on screen that you probably like is totally fucked.
- It's a small thing, but in Episode 32, in which Tenma purchases a sniper rifle and follows Johan into a park to assassinate him, the first time Tenma sneaks up on him and lines up a shot, Johan turns slowly and LOOKS RIGHT AT HIM. AAAGH.
- Nina freaking out when she starts to remember the day she was taken from her home. At one point, it's accompanied by a hallucination of Franz Boneparte with a massive Slasher Smile on his face emerging from the door and beckoning to her.
- Peter Capek is pretty creepy in those flashbacks too. "Keeping secrets from him is definitely a no-no..."