She only has to kill the Prince because the deal was if he fell in love with her she could be human forever, and he didn’t. By which I mean, he was a good person and genuinely nice to her, but he didn’t fall in love. He fell in love with someone else, also perfectly nice - not the seawitch in disguise, fu Disney. The Mermaid is told she can only return to the sea now if she kills the Prince. She goes into the room where he and his lover lie sleeping and they look so beautiful and happy together that she can’t do it.
That’s why she kills herself. And because it was a noble act she returns to sea as foam.
One moral of the story was that women shouldn’t fundamentally change who they are for love of a man, and in theory Han Christian Anderson wrote it for a ballerina with whom he fell in love. She was marrying someone else who wouldn’t let her dance.
