While not especially horrifying, the fact that Ariel is only 16 throughout the movie is pretty weird.
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, which is to the right. Obviously.
Several things about Ursula. 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". 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 spar of one of said ships and sinks, clutching the hull of the vessel with her tentacles in her death throes. The directors' commentary on the Special Edition DVD noted that they probably wouldn't get away with Chef Louis (or Ursula) in a modern film. Also, she has a great singing voice.
The skeleton of a deceased sea-monster that serves as Ursula's home was a corpse when she found it. Want to know where all that rotting flesh went? Well, there's a reason why she's so fat.
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.
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.
Ursula, while not a good character, was sane and composed during most of the film. However, when she transforms into Vanessa, its 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 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 taking all of Ariel's treasures and trinkets with it. The whole scene resembles something from the bowels of Hell!
The preceding argument is also pretty cringe-worthy. Hell, the music alone is enough to give you heart palpitations.
Triton: So help me Ariel, I will 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.
Let's be honest here, just Triton being angry is terrifying by itself.
Sebastian seeing the cooked seafood, ending with a dramatic sting on the stuffed crab.
The first time Ariel meets Flotsam and Jetsam with their creepy, echoing voices.