Well, in the story, the sea witch is a very dark part. She tricks the little mermaid into giving up her most precious thing, her voice, in exchange for legs. And the little mermaid's longing for the prince, which might lead to her destruction if things don't go right, is also a bit of a dark aspect. Moreover, the underwater world she leaves behind has its own rules and she's sort of defying them in a way that could end badly for her.
One dark element is the mermaid's sacrifice. She gives up her voice to the sea witch, which is a huge loss. Also, she endures great pain as she tries to walk on land with every step feeling like knives. And the idea of her potentially losing her soul if the prince doesn't choose her is quite dark.
The Little Mermaid story has several dark elements. Firstly, the mermaid's transformation into a human is not a happy - go - lucky process. It involves a lot of pain and the risk of failure. The deal with the sea witch is a very dark aspect as it's based on deceit. The mermaid is in a vulnerable position, having to rely on the prince's love to survive. If he doesn't love her back, she will die and turn into sea foam. This whole concept of her fate depending on someone else's love is rather dark and shows the harshness of her situation.