The bald vampire doing Maudette Pickens. *shudder*
Malcolm, Diane and Liam having that upside down body in their house, blood streaming from her headwounds into a bucket.
Malcolm: She felt so bad about what Jerry did, she made the ultimate sacrifice.
Bill: You all make me sick.
Diane: You used to be fun!
Vampire glamour is already a little creepy because it robs one of their free will, but it becomes both this and pure Paranoia Fuel when you think about it another way. Not only can a vampire control your actions, they can selectively edit your memories. Which means you could have been raped, fed on, or both and you would have absolutely no memory of it.
Tara's exorcism. She saw a little girl version of herself, which she had to stab, after which there was blood on her knife. It becomes even more creepy when Maryann reveals she was drawn to Tara because of this ritual, and the child-Tara with the big black eyes was what Tara looked like in Maryann's vision.
Eric killing Royce the redneck by ripping him to pieces.
And that scene manages to become even more Squick when Lafayette pulls Roy's metal hip out of his severed leg and frees himself with it.
Sookie being attacked by that creature who turned out to be Maryann. Cue the scene in which Dr Ludwig removes the poison from her incredibly deep wounds by running her fingers through the gashes.
Sookie's incredibly visceral screams throughout the sequence throw this trope Up to Eleven.
Heck, Maryann's entire trick of turning people into black-eyed zombies could qualify. It was Played for Laughs with a few characters, but to see Eggs and Tara beat each other up than have rough sex while Maryann's giggles with joy was disturbing in it's own right. Rutina Wesley performs some brilliant creepiness fuel as Hypnotised!Tara.
Tara: He's coming. And he's gonna kill all of us.
Maryann's wedding, beginning to top - from the bridesmaids theme, to stabbing Sam, to Sam in bull form ripping Maryann's heart out.
Bill and Lorena's sex scene. Squick.
Pam getting herself a rather extreme peeling. It's supposed to be hilarious but... OUGH!
Lafayette's scary visions after Jesus awakens his inner witch and makes him into a medium. For one, he sees Sam with bloody hands, whispering a threat in a low voice, making the usually comforting character very disturbing and scary, and then he manages to witness the ghost of Rene grasping Arlene's throat and saying he's inside her.
The spirits in season 4. They jump at the viewer in a very unpleasant way. Something about the way those shots are made (blurry and sudden) is reminiscent of The Slender Man. How's that for creepy and disturbing?
In hindsight; Bill having sex with Portia Bellefleur, Andy's sister, who turned out to be his great-great-great-great granddaughter. It became even worse when Portia actually wanted to continue the relationship.
Everything about Nigel Beckford. Hideously deformed face (presumably from decades of well deserved Cold-Blooded Torture by the Authority?) Check. Creepy, high pitched voice? Check. Eats Babies? Double check. It gets worse during the climax of episode 7 when we actually see him devouring a child.
And he's seemingly still on the loose. Sweet dreams.
Bill and Eric get silver pumped into their veins. This is like a human getting a whole circulatory system full of acid, except their healing abilities keep them alive and burning.
Tara's attempted suicide via tanning bed. You'd think she could have come up with something a little less long and painful. Not to mention her horrific screeches of pain while frying in the ultraviolet light. Urrgghhh...
Then there's poor Emma. Imagine that you're just an eight-year old child, kidnapped while your pack stands by and does nothing to help you, taken to a building full of vampires. Imagine that your only chance of surviving is to remain in your wolf shape indefinitely, and if you don't, the vampire who 'owns' you won't be happy. And not only that, there's a scary vampire with a melted face, who eats babies and children. Imagine being terrified of the vampires who took you, but even more terrified if both of them aren't around, and you shift back at the wrong time and in the wrong place, and he's there...
Bill's transformation after he downs the entire vial of Lilith's blood is the stuff of nightmares. The horror begins as his entire body contorts and twists as blood begins pouring out of every orifice and only becomes worse as the skin starts melting off his body. All of this is before he simply collapses into the typical true death goo...all before rising up as a Humanoid Abomination and letting out an inhuman screech, similar to that of Lilith.
His entire exchange with Sookie before drinking the blood counts as this too, especially for those who were expecting Sookie's arrival to return Bill's sanity to him. Seeing the way Bill spoke so harshly to her, as well as his quiet and measured Motive Rant about why he had taken the Sanguinista path suddenly brought it home that everything is not going to be alright.
Supplimentary comics make this scene—-as well as Bill's entire Face–Heel Turn somehow worse. While we already knew Bill was an agent of the American Vampire League and the Authority, we see that he actively took out a Sanguinsta leader, the ancient "Bloody Mary" Stuart. This was in order to abort her planned revolution on the eve of the Great Revelation,something that would have been a disaster.
Godric called Lilith a "godless god." She may well be a god/powerful being of vaguely celestial origin, but that doesn't mean that she's capital-g God, or has anything to do with such an entity. But, whether an in-universe deity or not, something brought Bill back from a true vampire death, making Lilith very powerful and scary, whatever she might be.
While relatively mild in comparison to the above examples, Jason's recent hallucinations of his and Sookie's parents broaches into this territory. In fact, it isn't so much the hallucinations themselves—if that's what they are— but how they seem to be slowly goading him into becoming more and more intolerant and racist towards the vampires—including Jessica. What makes it even creepier is that no one seemed to really notice his sudden change in behavior, except for Sookie, and even that was sort of a "pull yourself together" moment. Justified as they were in for the fight of their (un)lives, but still...
Considering the truth about his parents, it's actually implied that they weren't hallucinations at all, but their actual spirits goading Jason into a vampire-killing frenzy due to their own hate of the species.
Rosalyn's True Death. Fly!Sam flying into her mouth and shifting back into human form so she combusts? Squick.
Alcide eating JD to become the new packmaster, an act that disgusted him last season. And he doesn't even bother to shift first!
Building upon the above, the absolutely brutal attack on Nicole's friends by Alcide's pack.While most of the massacre itself is mercifully left to the imagination we still hear their screams and are treated to at least one unfortunate guy being pulled down by a transformed werewolf.
Plus the idea that Alcide, who previously put right and wrong above things like pack and species, and was the closest thing the show had to a paragon of morals is now willing to track and murder an innocent girl because she saw and knew too much. It shows just how much of a corrupting influence his new power has had on him—as well as Rikki, who's growing more insane by the moment.
Bill uses his godly powers to manipulate a prostitute's body so that she is in perfect position for him to siphon all the blood in her body out of her mouth, leaving her a withered husk. Bill's utterly blank and empty expression while doing all of this only adds to the horror.
The sound of every bone in her body breaking as he forces her to bend and move backward in the most unnatural, horrifying way possible. And the worst part of it is she's alive throughout the whole ordeal, watching her own blood pour from her own mouth into a dead-eyed vampire, like a scene right out of The Exorcist. And it's the last thing she ever sees before she dies and then visibly withers.
Episode 4 of season 6 is a huge one in terms of Adult Fear. Andy's finally found himself loving being a father to his four fae daughters...and then they're kidnapped by Bill and Jessica. He spends the whole episode out of his mind with worry, and then when Jason tells him that faeries are vampire catnip, he has a moment of pure horror before he goes tearing off. And unknown to him, it's already too late...
Related to the above, Bill's creepy smirk in the previous episode when he realizes that he can find an alternate source of fairy blood within Andy's daughters. Not so much the expression in itself, but what it means.
Things go further downhill when the ghost of Sookie's father leaps into Lafayette and attempts to carry out this mercy killing while Sookie screams in terror.
Sookie tearfully begging her father not to kill her before he plunges her head into the lake...
Seeing the effects of hepatitis V in full glory. It's a slow, painful death for any vampire who ingests it, complete with blackened veins and finally melting into a steaming puddle of chunky soup. Poor Nora...
Billith again in episode six and seven. Imagine the sheer terror of seeing a vampire walking around in daylight, coming into your home uninvited and threatening your child (if you're poor Andy), immune to wooden bullets on top of that. Imagine the realization that the primary weapon humans have against vampires - sunlight - might not be absolute.
The cliffhanger Season 6 ends on. A small group of feral, blood-covered HEP-V-infected vamps appear like wraiths from the woods, heading towards the jolly town social, including most of the Main Cast. Then more appear... then more... and more... until a whole horde of them are marching towards the buffet. Holy. SHIT!
Season 7 expands on the HEP-V vampires. They are feral, near-psychotic beasts even paler and sicklier looking than an average vampire with Tainted Veins all over their bodies. Because they are facing a slow painful death anyway, their attitudes range from sociopathic to outright crazed.
What's even worse is that packs of them are now roaming the country, killing anyone they can find and draining them of blood with no self-control and only the most minimal consideration for secrecy or finesse, and it's later revealed that the pack which attacked Bon Temps wiped out an entire town.
The Apocalyptic Log journal from said destroyed town. The tone of the starts out so innocent and cheerful as the writer describes her eerily familiar first love with a vampire and then ends as the poor girl approaches the Despair Event Horizon, contemplating suicide as the pack of rabid vampires beisige the town, killing people one after another.
The entire atmosphere of Season 7, really. It has all the trappings of a Zombie Apocalypse, except the "zombies" are fast, strong and worst of all smart.
Even though Sarah Newlin's a horrible person, her final fate - chained up in Fangtasia's basement and pimped out as a 'fresh from the tap' cure by Eric and Pam, completely insane and hallucinating her dead husband - is extremely grim.