There’s a book all about this

She’s the only antagonist in Disney that gets the hero to sign a contract. Ursie was robbed of her crown.

Stop. Just let them be evil. Let them all be evil. None of them are misunderstood. I hate this Disney shit where they try to justify why they are evil and somehow it’s the good character’s fault. Naaah. Stop that foolishness.

Because she's not misunderstood. There's a minor implication that she and Triton were involved in some way, but they separated because she was already doing the things she does during The Little Mermaid.