Wait no, on the original story it's explained that people on land has a soal and go to heaven when they die, but mermaids don't and turn to sea foam when they die. The witch and the little mermaid made a deal that if she married the prince, she would gain a soul, live on land and later go to heaven. But if he married another she would turn to sea foam, which means she dies. She has a chance to escape this by killing the prince, but she choses to die instead. So in the Disney version she was curious about the life in land itself, but the original mermaid was mostly driven by the desire to have a soul.
In the original the seawitch is neutral or kinda bordering to bad, while she is straight up evil in the Disney version. I thought it was interesting that she was an ally to the mermaid in the manhwa's version.
There is also a twist to the original story that makes it less sad, but it's too complicated to explain.
It wasn't the witch that allowed that, but the spirits themselves that were moved by her sacrifice. The witch would allow her to live as a mermaid if she killed the Prince, but that was after her sisters had pleaded with her and cut their hair off as payment.
For the queen, I never even thought about it. Interesting theory.
Although I love the original "The little mermaid" I think that's such an interesting twist on the fairy tale!