Over the course of this remarkably short story, the Little Mermaid has her tongue cut out, finds that her new legs grant her stabbing pain every step she takes, stands over her sleeping paramour with a knife, ready to stab him to death so that she can drip his blood onto her feet, commits suicide, and then cries and gets an extra day added to her time in Purgatory every time a child commits a bad deed.
While more of a neutral party than Ursula, the Sea Witch and her garden of polyps are frightening. The Witch herself is a devil ray, and the polyps capture and strangle Marina. Fritz saves her by using a mermaid skull as a makeshift weapon.
Jenny the cat wants Marina dead for no real reason. She succeeds.
The scene near the start where Flounder Ariel are exploring a shipwreck. Flounder is afraid, knowing there are sharks around. The tension is built up a bit before we actually see the shark and the voices and music are mostly quiet. It's like two characters innocently exploring while a killer lurks nearby, watching them. We get a false scare when Flounder sees a skeleton and screams. The music starts picking up after the shark swims by behind the friends a second time. When the shark finally attacks the music bursts in with Flounder screaming bloody murder the whole time. It barely seems like a shark at all, but rather a bloodthirsty monster. Though considering most of the sea life we see are very anthropomorphized it could just be some psychopath.
The shipwreck/fire scene can be pretty frightening, especially when lightning strikes one of the sails, setting it on fire. Then the ship crashes into a gaggle of rocks, sending crew overboard. As the crew makes it to the lifeboats, Eric realizes that Max is still on board—the shot of the terrified dog, frantically barking while surrounded by deadly flames doesn't help. Then, Eric risks his life to save Max and gets caught in the ensuing explosion and would certainly have drowned had Ariel not rescued him.
When Ursula shows what happens to her clients who break their contracts — they get turned into these immobile, screaming, limbless worms stuck by the hundreds to the floor of her cave and then it happens to King Triton.
It happens to Ariel too, and in a more horrific way: in order to scare and threaten Triton, Ursula begins to tranform Ariel then freezes the transformation, leaving Ariel as a mermaid-worm hybrid.
Consider the polyps in Ursula's garden for a moment. They went to her for help, and now are rooted in the ocean floor forever (at least, until Ursula's death), crying out and attempting to scare off more potential victims. How long have some of them been there? Do they continue to age in their reduced state? And imagine what their families and friends must have thought when they never returned...
This deleted scene shows the story of one of Ursula's previous victims. It's as creepy as you'd expect.
Ariel's (first) transformation into a human is horrifying. She is voiceless at this point, but her body is writhing in visible torment as something happens to her insides. Then her tail is ripped in two and hangs in shreds before two more flashes of magic turn the raw remainders into legs. Then she nearly drowns right there and then.
Several things about Ursula in general. She's a fairly grotesque octopus woman with pasty grayish skin. She eats live (and, as it turns out, sentient) shrimp as snacks. The entrance tunnel to her lair is carpeted with the shriveled bodies of her transformed former clients who "couldn't pay the price". Her interactions with Ariel and constantly trying to touch or grab Ariel with her tentacles feel reallycreepy. She conjures a pair of wraithlike disembodied hands to forcibly thrust down Ariel's throat and extract her voice. And at the climax of the film, she transforms into a giant, saw-toothed version of herself with a Stygian voice, uses the power of Triton's stolen trident to create maelstroms and raise up the rotting hulks of sunken ships, and finally is impaled on a spear of one of said ships, electrocuted by the lightning she causes, and sinks, clutching the hull of the vessel with her tentacles in her death throes. Afterwards, her remains are shown slowly sinking into her lair. The directors' commentary on the Special Edition DVD noted that they probably wouldn't get away with Ursula (or Chef Louis for that matter) in a modern film.
The skeleton of a deceased sea-monster that serves as Ursula's home was a corpse when she found it. Where did all that rotting flesh go?
During the song, "Poor, Unfortunate Souls", when Ursula sings, "It's she who holds her tongue who gets her man", she tosses a tongue into the cauldron. Easily missed, if you assume that it's something else.
The fact that Ursula's final scene is stolen from HP Lovecraft's The Call of Cthulhu doesn't help matters at all.
When Ursula is looking in the mirror, she essentially squishes a clam's insides up and uses it for her lipstick, meaning she was using clam guts/clam blood for lipgloss.
The whole concept of Ursula disguising herself as a shapely young woman is pretty unsettling, especially when she uses her disguise to hypnotize Prince Eric into doing what she wants.
Ursula's transformation from "Vanessa" back into her actual self, which involves the bodice of her dress ripping open down the front and her tentacles spilling out. As if that wasn't enough, she then (shudder) crawls over to Ariel and kidnaps her, all while laughing demonically.
The whole scene with Louis the chef gruesomely hacking up fish while poor Sebastian runs around in terror had to have made some people become vegetarians. It doesn't help that he's singing during most of it, making him look like a sociopath right before he goes off the deep end chasing a small crab. Honest Trailers outright calls the song "the traumatize your children song", and changed Louis's song to reflect on that.
Ursula, while not a good character, was sane and composed during most of the film. However, when she transforms into Vanessa, it's implied that she lost quite a bit of sanity (to the point of becoming a borderline Ax-Crazy) when turning into her, as she talks to her mirror in a manner similar to a schizophrenic, emits a psychotic grin when throwing a pin at a mirror's head with enough velocity to knock the mirror back, and most certainly kill a person had that been a human being, and her cackling.
Ursula's line to Ariel and the accompanying visuals: "If he [Eric] does kiss you [Ariel] before the sun sets on the third day, you'll remain human permanently! But... if he doesn't, you turn back into a mermaid and... you belong...to me!!!"
The entire sequence after Ariel signs the contract. The lighting changes dramatically from reds and golds to greens and blues with Ursula's Slasher Smile, and she begins chanting a spell over the cauldron. And then ghostly grotesque hands rise out, which basically are used as such when one reaches down into Ariel's throat to snatch up her voice. Think she felt it being physically ripped from her?
The aftermath of Flotsam and Jetsam's destruction, as well as Ursula's. First when Ursula says, "Babies! My poor little poopsies!", you can see an eye and remnants of the eels, and after Ursula's been destroyed and the trident's sinking, the remnants of her are sinking to the bottom, even as the merpeople she's tricked into making a Deal with the Devil change back, and quite possibly showering them in it.
Triton destroying the grotto and everything in it. He looks like a large, Satan-esque figure, and the whole scene resembles something from the bowels of Hell!
The preceding argument is also pretty cringe-worthy.
"So help me, Ariel, I am going to get through to you. And if this is the only way... so be it."
Listen to the scene without watching it, and it's even scarier. It sounds like a child begging her father not to beat her.
The music, during that scene; from the creepy build-up during the argument to the blaring horn stings and timpani drums when all hell breaks loose.
Just Triton being angry is terrifying by itself. There are Disney villains less scary than Triton when he whispers, "so be it".
The first time Ariel meets Flotsam and Jetsam with their creepy, echoing voices. Unlike many Disney villains, they are not ineffectual at all. In fact, they are almost as creepy as Ursula herself. In the stage version, they are even creepier with their Villain Song, "Sweet Child".
Eric under Vanessa/Ursula's trance is pretty disturbing, especially with his eyes glowing bright yellow and sounding completely monotone without any attitude or feeling in his voice.
Poor Unfortunate Souls starts out nice and slow, seductive but just a hint of danger but the second half ramps everything up as Ursula goes into her hard sell. Pat Carroll managed to make Ursula sound insane as she belts the lines.
The stage version
In the stage version, we get some pretty horrifying backstory on Ursula: She's Triton's sister, first of all, and they had about seven other sisters, all of whom Ursula arranged accidents for or outright killed through different subversive tactics. Then she moved on to Triton's wife, and was afterwards banished. All this in hopes of becoming the kingdom's ruler, and she damn well nearly succeeded.
The deal with Ariel, and the subsequent attempts to sabotage her? She wants to destroy everything brother dearest loves, just as she herself had everything taken from her.
The stage version also gives her an additional villain song, I Want the Good Times Back. What are these good times the song speaks of? Why, gleefully mutilating, murdering, and eating her subjects of course!
Its post-Broadway replacement, Daddy's Little Angel, is equally creepy, describing how she murdered her six older sisters out of jealousy.