Over the course of this remarkably short story, the Little Mermaid goes through some terrifyingly agonising situations both physically and mentally. She starts by having her tongue cut out as payment for becoming a human, only to find out that her new legs grant her pain that can only be compared to stabbings for every single step that she takes. All of this comes crushing down when it turns out that the prince whom she loved and who was part of the reason for her whole ordeal, has fallen in love with another and never really realised her feelings. This means that the magic will wear off and she will die and that the only way to avoid it is killing him. She ends up with a knife, ready to stab him to death so that she can drip his blood onto her feet and live but eventually chooses to make a Heroic Sacrifice. This fortunately ends up guaranteeing her an immortal soul and turns her for centuries as a daughter of the air who can earn a place in heaven, with a time of service that can be extended every time a child misbehaves.
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 who causes storms wantonly, leading the Prince to almost drown, the polyps capture and strangle Marina, and the garden is filled with the bones of dead merpeople. 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 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 six 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, with Ursula describing how she murdered her six older sisters out of jealousy.