Those dismembered dolls, especially how they appear in brief snippets without explanation... and Mei's insinuation that the dolls are trying to suck out your soul doesn't help.
In Episode 3, things go from zero to sixty in about three seconds. To wit, Yukari Sakuragi sees Mei in the hall, panics, and flees down the stairs, whereupon she trips and gets Impaled with Extreme Prejudice on her umbrella. And it's shown in graphic detail.
The worst part is seeing her fingers twitch in her own blood.
Then there's Episode 4... not only does it recap Sakuragi's death in the Cold Open, Mizuno is killed in a malfunctioning elevator. They don't show her face being crushed in as she falls down once, but three times!
In Episode 5, we get a close-up of Tabakayashi's face as he suffers a fatal heart attack. His eyes are practically bulging out of his head.
The ending of Episode 6. Mr. Kubodera stumbles into the classroom, apologizes to the students for allowing the Calamity to start, then howls like a crazed animal as he pulls out a giant butcher knife. Cut to credits.
The class' Freak Out at their teacher's suicide. Genki Girl Aya Ayano gives a Big "NO!", half the class bolt for the doors, one poor girl is in hysterics and holding her head, Yumi Ogura sits on the floor having a Heroic BSOD, ill student Daisuke Wakui throws up, and in the background, Kazama just sits at his desk covered in blood looking traumatised.
The dream sequence. Specifically, the face melting part. The way Kouichi screams during it is in no way better.
For the most part, Episode 8 is a Breather Episode. Then the ending happens. Nakao suffers an aneurysm while swimming after a beach ball. Then a boat speeds by, not seeing him as he bobs in the water, and chops his body to pieces. It stains the water red. Then we get to see his limp, bloodied body launched into the air. The accompanying music does not help.
The atmosphere of sheer panic as the others think something has happened to Nakao. It makes the above that much worse when it actually happens, because at first, everything seems to be fine. They all thought they were safe outside the town, but the Calamity can happen anywhere now...
Sakikabara's dream from Episode 9, where he sees the bodies of his dead classmates, who all blame him for their deaths.
It's your fault!
During the flashback to the year the Calamity was stopped mid-year, Matsunaga describes how the first student to die on the trip was struck by lightning. The kid was standing right in front of him, and we get to watch the kid be struck. Matsunaga even describes what his burning flesh smelled like.
All of Episode 11. Higlights include:
Everyone puts on a Nightmare Face with Slasher Smile when Takako announced that Mei might be the dead one, and then they all slink out to kill her...slowly and quietly.
Takako popping up behind Kouichi and saying "It wasn't me" calmly is also an extremely creepy Jump Scare.
Kazamiis still alive after he was pushed off a balcony.
Teshigawara and Mochizuki opening one of the hotel room doors to find the insane, blood spattered, knife-wielding old lady who lives in the house, who then proceeds to try and stab them. Then there's the later Jump Scare when they try to enter the fire exit, only to find a dead body already in there, staring at them wide eyed.
Ogura's psychotic breakdown. She joins the chase to kill Mei, and tries to follow her and Sakikabara out a window, determined to avenge the death of her brother, panting and screaming and waving around a knife. When she thinks she has Mei cornered, she has a horrible, blood-spattered Slasher Smile.
The dining hall at the lodge has caught on fire, and two boys in the class smell the smoke. One of them opens the door to check it out, and is immediately engulfed in flames. After the gas in the kitchen causes two giant, exploding fireballs, we cut back to the student's charred remains and his friend looking on in horror.
Episode 12: Kazami systematically slaughtering his classmates to figure out who the Dead One is. He casually and calmly strolls up behind a girl who has run to Sakikabara for help, and stabs her right through the back of her neck. Then he decides that Sakikabara must be the Dead One, and loses his shit trying to kill him.
Much of the Paranoia Fuel both in the show and in a meta sense. As much of the early deaths look like gruesome, but every day accidents. It gives anyone watching the impression, that it's not as accidental as they think it is.