This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Animal Farm
The entire book is nightmare fuel if you happen to be a farmer.
"It was a pig walking on his hind legs." One edition of the book has this sentence as the first one on a left-hand page, forcing the reader to turn the page before finding out exactly "what Clover had seen." For someone not expecting it, it can be a Wham Line that can force the reader to put down the book and walk away to catch his breath.
Napoleon deciding to have the dogs chase out Snowball, who at that point had been the Reasonable Authority Figure and Token Good Teammate of the pigs. The scared expression he has and the implications that he was mauled to death, makes the scene all the more disturbing.
The Kangaroo Court that Napoleon holds to execute falsely accused animals with the aforementioned dogs eating them as punishment.
It gets so bad in the 1954 film, that the crow that had nonchalantly watched Snowball's death at the hands of the dogs turns its back on the scene, being absolutely horrified at this cruelty.
The infamous final scene of the book, where the humans and pigs all start resembling each other in a way that nobody can tell them apart anymore.
Napoleon claims that Boxer is being sent to the vet when he's injured and can't work any more, but really he's sending him to the slaughterhouse because he's of no more use to Napoleon. Imagine how kind, trusting Boxer must have felt when he realized where he was really going and how the pigs had betrayed him right before he was killed.
The ending of the 1954 animated film, which sees the pigs on the receiving end of a Bolivian Army Ending, seesaws between this and Crowning Moment of Awesome. On the one hand, there's the fact that the filthy swinish bastards are finally going to get what's coming to them. But on the other hand, the scene itself, with hordes of red-eyed animals ripping and tearing their way into the house like a swarm of zombies where their former oppressors, the pigs, are cowering in terror at their strongly implied imminent, terrible death at the hooves, paws, claws and mouths of the animals they once lorded over... it's chilling.
The Public Execution scene for the chickens, sheep and ducks. The 1999 version didn't show much except a chicken in a noose. To install fear into the rest of the animals, Napoleon and Squealer recorded the murders and showed them to the animals. This is supposed to represent Joseph Stalin killing some of his subjects during his reign.