The ending credits for the beginning seasons. It first shows flames with the main characters as Tarot cards. Then as the flames begin vanishing and the credits come to a close, there's a very creepy Voice Of Legion.
In "Sickly Sweet", just to scare off a couple and real estate agent, Grim transforms a tiny, innocent flower into a giant, plant-monster... thing that begins to trash the ever-living hell out of Billy. It's made much worse as Billy screams "IT'S DEVOURING MY NOOOOSSSEEE!!!!!". The real estate agent responds with some mood-clashing Casual Danger Dialogue:
Real Estate Agent: Looks like we've got a bit of a weed problem.
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 "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 neverheard of in the show again.
Little Rock Of Horrors with the Audrey II-like space rock that required Billy to get it human brains.
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 has an illustration of Billy on it who 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 real Cthulhu (the one playing golf is really one of his Star Spawn) shows up and eats Billy at the end, meaning they failed to stop him.
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 Terminatorhomage, complete with its insides showing and says "Of course I forgive you, Pud'n! 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 its cargo, agiant pillow, hit with thecreme 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 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 Scare Chords 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 Mrs. Doolan really was a ghost.
Mandy: Mrs. Doolan beat Grim. She beat death.
Also, the possibility this episode is where Bleedman took inspiration for Grim Tales from Down Below. Mandy did the same thing Ms. Doolin did. Only rather than just defeat Grim, she married him, thus taking the power of Death for herself.
From the Billy and Mandy's Big Boogey Adventure movie. The main cast and Irwin are put into a sort of dream loop by Boogey. 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 Irwin as part of its body 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.
In one of Mandy's intro quotes, she just walks on screen, points at the viewers, and lets out a demonic screech. This scene, or at least one like it, was actually a Shout-Out 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.
"A Dumb Wish", in which Mandy wishes that everyone in the world except her would go away.
The ending to "Scary Poppins", which shows Mandy apparently controlling the minds of everyone in Endsville.
Mandy: This is my world. I'm just letting you live here.
The trailer to "Wrath of The Spider Queen". I haven't found it anywhere on the internet so far, but it's just unnerving. The Spider Queen's singing as Grim says the title of the special(with him also giving a good punch to the fourth wall in the actual special) is particularly unnerving. Made me frightened when ever it came on.....
Billy's nightmare in "Tickle Me Mandy".
The ending to "Spider-Mandy", where everybody in town has become a mutant spider creature. This proves to be in-universe nightmare fuel to Billy.
Spider!Gladys: Do you want to breakfast...BEFORE I EAT YOUR HEAD!?