Even though this is a Shonen Jump manga published alongside the likes of One Piece, Bleach and Naruto, make no mistake: there's enough Nightmare Fuel to fill a whole classroom. Heck, it wouldn't be a Shonen Jump title without this trope.
In accordance with Nightmare Fuel policy, all spoilers are unmarked.
- At the start, we have the Assassination Classroom's own nonhuman professor, Koro-sensei. Imagine having a strange creature as a teacher. Now imagine that under his cheerful facade, you know that whatever he is, he's immensely powerful and can (and will) destroy the entire planet in one year, unless he's dead. And the first proof of his power? Thanks to him, about 70% of the moon has been defaced, turning it into a permanent crescent. And he's very hard to kill, if not impossible. Normal bullets don't work on him, and he can reach Mach 20 easily note . Luckily his weak point (some substance which can be used for BB pellets and knives) is revealed from the beginning, but good luck trying to kill him with it. Thankfully, he very quickly becomes Nightmare Retardant the more time is spent with him.
- Pitch Black: Furious. Koro-Sensei truly angry is terrifying, in both appearance and intent. In five seconds he gathers the name plates of his students' houses and the houses of all of his students' friends and family. He knows where everyone lives. He gave his word that he wouldn't harm any of the students, but if he wanted, he could kill anyone else. There's nowhere to hide.
- Karma's memory of his old teacher is metaphorically shown as said teacher disintegrating down to his skeleton. Brr.
- In the Kyoto Arc, Kayano and Kanzaki are kidnapped by a gang of delinquents and blatantly threatened with rape and torture. Their leader makes some very creepy faces while happily talking about how he and his buddies have "messed up the people who think they're elite" (they have no idea that the Class E students are treated like absolute shit at their "elite" school). These are high-school-aged boys acting like hardened Yakuza thugs.
- Takaoka Akira may look like a fun-loving instructor who thinks of E Class as his own children at first, but he is really a sadistic military dictator who is willing to make Class-E's life a living hell just so that he can have superiority over Karasuma, and isn't afraid to hurt or kill anyone who tries to disobey their "father".
- When Karasuma stops him from harming the students, Takaoka responds by setting up a Morton's Fork by having Karasuma either choose having an unexperienced student hit him with a real knife, planning to deliver a No-Holds-Barred Beatdown on his victim or have Class E obey Takaoka unconditionally. If Nagisa hadn't won against him, Takaoka would've destroyed Class E in both body and spirit.
- After his initial defeat, he returns in the Assassination Island arc, having been driven mad by his defeat at the hands of Nagisa:
- Episode 21 begins with him watching the kids and the teachers on security cameras and saying that once he kills them all and destroys Koro-sensei, maybe he'll use the reward money to buy more kids and fill them with the virus and watch them die. All this as he's scratching skin off his own face, which he's been doing since he was removed from the school after being defeated by Nagisa.
- The scratching off his face and the Thousand-Yard Stare doesn't even cover it: Once he forces them all to the roof, he says that his assassination plan would've gone well if Kayano and Nagisa had gone by themselves: First, filling his bathtub with Anti-Sensei BBs and burying Koro-Sensei and a tied and gagged Kayano in them and covering the whole thing with cement, leaving Sensei to blow up everything if he wanted to get out but knowing Koro-Sensei loves his students, he would choose to die in there with her. As we know, the plan went wrong for two reasons: one, the non-infected kids banded together to save everyone, and two, Kayano would've melted away when she was placed in the bathtub or would've blown up the entire place to get herself out of there alive. Not that she would've cared about leaving Sensei in there.
- While thoroughly deserved, we also have Takoaka being defeated by Nagisa again and just snapping completely, first through a Freak Out of epic proportions as he sees Nagisa smile at him once more, and then through going totally catatonic as he is carried away on a stretcher, wrapped in a straightjacket.
- The "God of Death". He's capable of disguising as anyone, or even anything, as shown when he attacks a hitman after (seemingly) emerging from his own shadow. And if you need more proof, his camouflage skills are so high he appears in front of the students without disguise as an humanoid silhouette with only his two eyes glowing like a pair of spotlights. He even skinned his own face to improve his disguises. Yet, somehow, his hair, eyeballs and everything else remain in place, even though he's got no facial muscles anymore! Forget Two Face, this guy is even scarier! And then there's his "Death Scythe", a small, hidden gun inside his index finger. This guy can kill you just by doing a single "Bang!" gesture. This is when Crazy Is Cool is mixed with a villain hitman.
- Nagisa's mother! The woman sees Nagisa as an extension of herself, dictating where he'll go in his future and what job he'll have simply from what she wanted and failed to achieve in her youth. What makes this worse than a typical Stage Mom situation is that she denies Nagisa's own gender, forcing him to keep his hair long (hence his pigtails) and even makes him wear dresses to give him "advice" on how to dress in the future. Whenever he shows the slightest deviance from her plan, she flies into a rage and attacks him, emotionally blackmailing him into going along with her. You know how Nagisa's so good at reading and observing others? That's why. This is made all the worse when, instead of questioning why he wants to stay in E class, she decides the most logical route is to drug him, drive him to Class E at night, and then set fire to the building so he'd be forced to beg the chairman to let him in to the school again! If it weren't for an assassination attempt, things could have gone very hairy indeed. In other words, the appearance of a murderous hitman made the situation less scary.
- The flippin' principal. Asano Sr. is not right in how he treats everyone - including his own son - callously, and that centipede motif of his... When he completely snaps, this gets ramped up to eleven. He moves to destroy the Class E Building WHILE CLASS E IS STILL INSIDE OF IT, intends to move them to a new school building that should just be called a Hellhole Prison, and announces his intent to fire Koro-Sensei. To reiterate, he just stated that he intends to completely terminate Class E, the only thing stopping Koro-Sensei from destroying the world. And all of that is because Class E and Koro-Sensei have proven his educational model does not work. And this is mild compared to what he's done to some other people. Remember that bum Asano gave some money to? That was one of the boys who bullied one of his students and drove the student to suicide. Really makes you realize just how badly Asano was affected by the student's death. He could've used all his intellect to simply kill them and make it look like an accident. Instead? He got them hopelessly addicted to gambling, robbed them of any particular drive except to gamble, and ruined their lives so that they became dependent on the allowance he gives them. Really puts a lot of other things into context. Do not make Asano mad by harming the people he cares for.
- Six words: "I love you, Koro-sensei. Now die." Kayano then reveals she has a set of tentacles on the nape of her neck. Not tentacles doubling as hair, like Itona, but black tentacles connected right to the nape of the neck. Then, when the first attempt is unsuccessful, Kayano meets Koro-sensei for what is essentially a Suicide Attack/Self-Destructive Charge. But that's not the creepy part. She's sweating so much she looks sick, looks more and more deranged the longer the fight goes on, and is explicitly stated to be losing her mind.
- Chapter 130 depicts the harmful effects of Kayano's tentacles on her mind using an imagery of Kayano's brain being wrapped in barbed wire.
- How Koro-sensei got close to the woman in his flashback. He was captured to be used as a test subject in a potentially hazardous experiment, and she was the one who interacted with him personally through a pane of reinforced glass. That's not the scary part: the scary part is that the head doctor, her fiance, knows that if Koro-sensei lost control or became too enraged, his new powers would enable him to get through that glass and kill her with ease. The doctor even outright calls her a potential sacrifice! She was close to him because the other researchers wanted someone expendable in that position.
- Koro-sensei explains in detail what happened to the moon. It wasn't him. It was a mouse implanted with his cells that exploded when the mouse reached the end of its life. If Koro-sensei isn't killed, he might self-destruct and take Earth with him!
- Nagisa after Takaoka blew up the antidote, dooming the students who were infected. His eyes were glowing green, for crying out loud.
- On that matter, Nagisa's snake motif. It shows up several times during the series, when Nagisa uses his bloodlust: Karasuma senses it during a training exercise before Takaoka shows up, and is scared out of his wits. Takaoka sees it during his first fight with Nagisa, where he hallucinates a giant snake strangling him as Nagisa makes his attack. Karma perceives it in the classroom civil war, where Nagisa appears surrounded by demonic snakes. Perhaps the worst of all is when Nagisa kisses Kayano to break her free of the tentacles' influence, where it's portrayed as a rabbit being strangled to death by a giant snake! It's unclear whether this is just an artistic portrayal of the bloodlust, or a hallucination, but either way it's pretty scary.
- Some context for what causes Karma to notice Nagisa's bloodlust. A little bit before Karma envisions Nagisa's bloodlust, Karma goes to look for the table Nagisa is sitting at (in the scene, which is set before the series begins, Karma and Nagisa are eating at a fast food joint) and it's Nagisa's rather innocuous attempt of getting Karma's attention that causes Karma to notice Nagisa's bloodlust. What actually happens is that Nagisa taps Karma's back to get his attention. What Karma seemingly perceives to have happened is, in a similar style to how the snake motif is presented, a silhouetted version of himself is shown being stabbed in the stomach with a katana! Here's a picture of that by the way
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- Some context for what causes Karma to notice Nagisa's bloodlust. A little bit before Karma envisions Nagisa's bloodlust, Karma goes to look for the table Nagisa is sitting at (in the scene, which is set before the series begins, Karma and Nagisa are eating at a fast food joint) and it's Nagisa's rather innocuous attempt of getting Karma's attention that causes Karma to notice Nagisa's bloodlust. What actually happens is that Nagisa taps Karma's back to get his attention. What Karma seemingly perceives to have happened is, in a similar style to how the snake motif is presented, a silhouetted version of himself is shown being stabbed in the stomach with a katana! Here's a picture of that by the way
- There's a scene where Shiro is discussing his latest plan to kill Korosensei, and a mouse from one of his magic tricks crawls up into his mouth. And he eats it. In light of that, it's probably not too surprising to learn he's resorted to injecting himself with tentacle cells...but it is also very, very bad news.
- During the final battle with the God of Death, Kayano lunges at him to land a blow, only to get impaled by one of his tentacles. At this point, we finally see Koro-sensei's reaction to one of his students being hurt like this. Yes, he gets angry, but his standard black-coloured anger won't cut it here. He gets so, so much more enraged than he's ever been that he completely ignites into a blinding-white glow with a red Battle Aura, roaring his lungs out the whole while. Sure, he's able to harness it into a battle-ending explosion, but seeing Koro-sensei - a kind-hearted and empathetic teacher who loves each and every one of his students - that flaming pissed is jarring indeed.
- Irina's childhood is both terrifying and tragic: The first snippet we get is her hiding from the people who brutally murdered her parents before her very eyes, crammed in a closet with the freshly killed corpse of her own victim and feeling as the warmth slowly leaves the body, at the age of 12. Later we see her being trained as an assassin, presumably at most a year or so later, with a particular shot of her stabbing an older man in bed as he's pushing her down..
