The crimes for which a lot of the inmates were incarcerated are often all too reminiscent of real cases. Atroe (F.F.'s original body) kidnapped a child merely to fulfill a childhood fantasy. Kenzou founded a religious cult (its ranks including some celebrities) and instigated a Jonestown/Heaven's Gate-esque mass suicide. Anasui caught his girlfriend in bed with another man and literally took them both apart.
Narciso Anasui in general is terrifying. His background establishes him as a maniac and a full-blown Yandere and his infatuation towards Jolyne is actually creepy, with him wanting to marry her after seeing her despite them having never talked before. His Stand Diver Down can "dive" into objects and people...then it can rearrange and warp the internal structure. The way he defeats Kenzou, although satisfying, was nightmarish. He warped the bastard's bones and flesh to turn his legs into springs, causing Kenzou to bounce around uncontrollably. One shudders to think just how far his Yandere tendencies towards Jolyne would've gone if her father wasn't Jotaro.
The way Gwess uses her ability is beyond messed up. Her Stand, Goo Goo Dolls, has the power to shrink people, kinda like Formaggio's Little Feet from Part 5, only if the victim disobeys the user's orders, the Stand appears again and rips them to shreds. And its user is a Yandere obsessed by having absolute control over her 'friends'.
She also forces her victims to wear animal suits like rats and parrots. What makes this horrifying you is that they aren't real suits. They're actual dead animals that had their organs hollowed out so she could fit her victims in. Imagine being shrunken down to the size of a rat, have it thrown on you, and being forced to wear it's skin. And if you happen to hate rats...
If you thought Steely Dan's ability to transfer damage done to him onto your grandfather was bad, imagine being targeted by Thunder McQueen's Highway to Hell. It inflicts all of Thunder's wounds on you, and unlike Dan, who uses Lovers as a deterrent, Thunder is actively and violently suicidal.
Foo Fighters' debut chapter. Hermes suddenly notices there are six people in their search group whereas previously there was only five. The prisoners begin to discuss the matter when their security bracelets begin beeping...as in, the bracelets that explode if they get too far away from the guard start beeping, and Atroe's actually explodes. After a brief scuffle with the Stand Foo Fighters, Hermes and Jolyne get back to shore and being demanding who the Stand user is and when no one fesses up Jolyne begins attacking everyone...and they begin melting, their faces elongating and black slime oozing out of their mouths and eyes as Foo Fighters reappears (as a Stand seperate from a Stand user, to boot). It then cuts to a shot of◊ the black-haired and shaved head prisoners as Foo Fighters drains their body fluids with the shaved head woman giving us a terrifying Glasgow Grin. Was it mentioned that the shaved-head woman had inverted colored eyes?
Sports Max's stand, Limp Bizkit. INVISIBLE, INAUDIBLE ZOMBIES! Need anyone to elaborate? What's better? He can turn himself into one if he dies. When he became one, he ate a woman and got covered in blood giving us this lovely scene.◊
Sports Max in general is an incredibly creepy individual. His Establishing Character Moment has him brutally execute someone by having them bite down on a chunk of wood and while he stomps the back of their neck. When Ermes' sister Gloria catches sight of this he whips around and delivers a hellish stare to her location. Later Gloria was found dead in a ditch, and its implied that despite the police finally managing to catch him, Sports Max managed to pull some strings to earn a mere five year stint for tax evasion and assault rather than the life imprisonment he should have gotten for murder. His death is brutal too- Ermes traps him in a prison sewage pipe and drowns him in filth. A monster like him deserved such a demise, but its still a horrible way to go.
Viviano Westwood was happily pounding on Jolyne while his left cheek was ripped enough to reveal some of the jawbone. Jolyne eventually had to try skinning him with his own toenail to escape.
The Stand Yo-Yo-Ma seems useless...but it secretes a flesh-dissolving acid from its mouth, and hides it by pretending to be affable and dim-witted. It fills Jolyne's tongue with holes to prevent her from warning Anasui.
Emporio, under the influence of Jail House Lock, being unable to move from an electrocuted puddle because he's stuck in a constant loop of remembering what he's supposed to do.
The Stand Sky High is this. It controls the fictional creatures called "rods", which move at speeds too fast to see and can drain body heat without even touching you. The effect of this and the way they're described are downright horrifying.
Heavy Weather, the forgotten ability of Weather Report, manipulates the ozone layer. It can use this to create a subliminal message which brainwashes its victims into thinking they've been turned into snails. It's not clear whether Your Mind Makes It Real is at play, or if it's all mental, but either way, Ermes, Jolyne, and pretty much everything around them are slowly transformed into snails... and then get attacked by shell-wearers, a type of beetle that eats snails. Overall, it's one of the weirdest yet most disturbing Stand abilities in the entire series.
Pucci's C-Moon has the ability to reverse an object's gravity. This represents in the object turning inside-out. It happens to Jolyne's hand.
The C-Moon sequence is particularly chilling because hundreds of civilians get caught up in the (three-kilometer!) range of its gravity shift effect. Most of them presumably end up dead or seriously injured. The scene with numerous cars tumbling down sideways down the road is especially creepy. Imagine that you're visiting NASA with your family one beautiful day... then suddenly gravity starts acting sideways for no reason, and you, along with a crowd of people all around you, start uncontrollably falling to your doom.
While it's hard to sympathize with him at this stage of the story, Pucci's demise at the hands of Weather Report is utterly brutal, even for Jojo standards. As he lays dying from Oxygen poisoning, Weather Report slowly pushes his bare knuckle against the side of Pucci's head, breaking his skull and causing his eyes to pop out slightly. When the beatdown begins, it's utterly savage- Weather Report smashes Pucci's head brutally, before following up by tearing off the side of Pucci's face with his bare hands, and finishes by ruthlessly smashing him against the floor so hard that his head explodes.