Maleficent had its misses as a recontextualizing of one of Disney's greatest villains, but fortunately, we also have this short story from Digger creator Ursula Vernon, explaining why the sea witch Ursula really took the Little Mermaid's voice—to prevent human colonization under the sea.
In "The Sea Witch Sets the Record Straight," Vernon presents Ursula as a wise—but wry—lady who can imagine just what would have happened if she had left Ariel with a voice:
I didn't take her voice for myself. I want to set the record straight on that, right up front. People got a lot of crazy notions in their heads, the way the story got around, and that was one of them.
I'm not saying I never did an evil deed—anyone who says they haven't is lying through their teeth—but I didn't take her voice for myself. I didn't need it. I've got a perfectly fine voice, thank you, trained by whale divas, and it's mine.
Seriously, you start stealing people's voices and using them yourself and pretty soon you don't know which voice is yours and which one's an echo and then you're mad and howling and people are standing around in caves during low tide asking where the screaming's coming from and someone else is saying "Oh, it's just some trick of the acoustics."
Go ahead, laugh. That trick of the acoustics is my Great-Aunt Meryl and you don't want to wait for high tide. I've seen her tear the head off an elephant seal. With her nails.
Best not to start down that channel at all, really.
No, I took her voice for two simple reasons—she was a twit and she was in love. I took one look at her and knew that she'd spill everything she knew in the pretty human boy's ear, and then where would we be?
It doesn't go so well when humans know about us, have you noticed? Ask one selkie if she's feeling happier now that she spent a decade on shore with some jerk who stole her hide off the rocks. (Sure, some of them think it'll be romantic—bull selkies aren't anybody's notion of charming, though they do have a certain over-muscled appeal—but it's not so romantic when you're spending your youth cooking and cleaning for an illiterate fisherman and bearing his brats through a pelvis that isn't nearly so accommodating as it used to be.)
I'd say "ask a Stellar's sea cow" but you can't, because they're all dead. And just try to find a sea mink. I was very fond of sea mink. They were inquisitive little devils and they made chirpy noises when you stroked them. I haven't forgiven humans for the sea mink. Or the sea cows, for that matter.
Do not get me started on the great auk.
Don't get her started, but do read the rest of the story on Vernon's website.