The Grim Adventures of Billy & Mandy/Nightmare Fuel

 has a large number of this, plus gross-out humor.
 * The so-called Christmas episode. Bad Santa? More like blood thirsty Santa. This must be the first time a show's Christmas special is scarier than its Halloween Episode, which is quite mild.
 * The episode where Billy and Irwin accidentally turn the Life Hourglasses of Mandy, Grim, and themselves upside down, and they all revert back into their pre-birth states and disappear at the end, to be creepy.
 * Nergal Jr. You don't want to mess with that kid. His "true form" is truly horrifying. There's also the shot of his tentacles coming out from weird fleshy holes in his back and head.
 * The episode where Nergal Junior takes over the teacher, and gives the class bully a fanged grin that slowly grows wider than her face? Bring My Brown Pants.
 * The "green-eyed kid" form he is usually seen on the show in isn't even his own original form, he stole it from the first person he met he met when he rose from the core of the planet. His original form is never seen by the viewer, but the original green-eyed kid is unfortunate to, and is outright terrified of it. The best part? The original green-eyed kid is never heard of in the show again.
 * Little Rock Of Horrors with the Audrey II-like space rock that required Billy to get it human brains.
 * Dream A Little Dream had scenes based on the "Night on Bald Mountain" sequence from Fantasia but not as scary.
 * In "Chocolate Sailor", Billy turns completely into solid chocolate after eating too many supernatural chocolates. If that isn't enough, Billy even begins to eat himself, saying to Mandy that "...it's not that painful" at one point, and continues to do so, reducing himself to "his big, chocolate head" near the end of the episode. At the episode's climax, the Chocolate Sailor gives Billy the antidote, but with a twist: it's in a box of assorted chocolates and Billy has to pick the right one. Since Billy is stupid incarnate (the antidote bar is called Antidote), he eats the whole assortment and promptly explodes. The episode ends with Mandy and Grim eating hot fudge sundaes, with them saying Billy is in a better place. The bottle of hot fudge syrup they have then says "I like chocolate!"; implying that they scooped up Billy's remains and dumped them into a chocolate syrup (and to add that Grim and Mandy are eating hot fudge sundaes...
 * The Cthulhu episode.
 * The Monster Clown episode.
 * "Wishbones".
 * Pud'n's part.(shudder) What really drove that one home is how realistic the bunny was drawn. When Pud'n's holding the cute bunny and it lets out a terrifying shriek as its face contorts. Cut to Pud'n's face looking like he pooped himself. And when the bunny rises out of the fire of the destroyed truck in an obvious Terminator homage, complete with it's insides showing and says "Of course I forgive you, because I love you... And I love you, to death." The epic Mood Whiplash (the truck had just blown up from having it's cargo, a giant pillow, hit with the creme filling from an eclair) really made the moment scary. It does just turn into standard cartoon violence for parts of it, but the fact that it goes from that to the said quote is horrifying.. that voice is just awful.
 * It was disturbing that Pud'n was searching through the garbage for candy.
 * The part where he rips the cactus out of his eye.
 * The character of Pinface considering his voice is done by John Kassir (you know him as the voice of the Crypt Keeper in Tales From The Crypt.
 * Most of "Who Killed Who?" is absolutely terrifying, especially if you haven't seen all of it. It's one of the episodes that use ScareChords more often than the normally cartoonish sound effects, adding to the horror even more. That's not even counting the Twist Ending, where it's revealed that
 * From the Billy and Mandy's Big Boogy Adventure movie. The main cast and Erwin are put into a sort of dream loop by Boogy. Mandy wakes up on a beach by the River Styx and says "Wow, I just had the craziest dream...". Billy (off camera) then says "Is it the one where you couldn't save us?" We're then treated to a FREAKY looking creature that has Billy, Grim, and Erwin as part of its bodies as it wails in a distorted voice, "Where are you, Mandy?"
 * In one episode, Grim falls for a goth girl and struggles to tell her that he is, in fact, the Grim Reaper. So how does he show her? By disturbingly rising from the ground while fire forms around him whilst he randomly takes on this slinky look akin to Slender Man with a horrifying frown throughout.
 * The ending to "Hog Wild". Even worse is that it comes out of nowhere.
 * In one of Mandy's intro quotes, she just walks on screen, opens her mouth, and lets out a demonic screech. This scene, or at least one like it, was actually a Shoutout to the alien screech from Invasion of the Body Snatchers.
 * Mandy smiling. Not her evil smiles, but a genuine grin. Considering her character, it's extremely unsettling.
 * The very end of Something Stupid This Way Comes. "And you. You will be my closest, most bestest friend of all!"