I've been thinking of one of the biggest differences between the different versions of Disney's The Little Mermaid: who kills Ursula. In the animated classic, Prince Eric kills her by impaling her with a ship's bowsprit, while in both the stage musical and the live action screen remake, Ariel kills her: in the musical by destroying her magic nautilus shell, in the movie by impaling her with a ship like Eric does in the animated version.
Which variant is better can probably be debated endlessly.
Pros of Eric killing Ursula:
*Ariel saved Eric's life during the storm near the beginning. By saving Ariel's life at the climax, he returns the favor. It's more balanced than having Ariel save him twice.
*Eric has experience as a sailor; Ariel doesn't. If anyone can steer a ship's bowsprit into Ursula's heart, "realistically" it should be Eric, especially since Ariel is in mermaid form. (This doesn't apply in the stage musical where Ariel destroys Ursula's magic shell, though.)
*Eric saving the day obviously gives him more agency and heroism. Without it, he becomes a more passive figure who chiefly matters as an object to motivate Ariel.
*Eric battling and killing Ursula to save Ariel is the ultimate proof of his love for her and his worthiness of all her sacrifices, both to us, the audience, and to the characters.
*Eric saving the day is proof for Triton that humans aren't evil after all.
(The above factor is what most people who defend the original film's ending seem to emphasize. It's debatable if it's really so important, though, or whether realizing that Ariel loves Eric enough to have sacrificed everything is what changes Triton's mind. Triton doesn't even see Eric kill Ursula, we can only assume Ariel tells him about it offscreen, and his dialogue with Sebastian the next morning is only about letting Ariel be free to live her own life, not about Eric. Although in the deleted alternate version of the ending, he does say "That human saved my life," I assume it was a deliberate choice to replace that line with dialogue focused on Ariel.)
*Related to the above, if Eric kills Ursula, than Triton's choice to turn Ariel human again can be seen as not just making Ariel happy, but rewarding Eric with Ariel's hand in marriage. If you feel as if the ending unfairly rewards Ariel for her deal with Ursula, then it might feel better to view it as Eric's reward instead.
Pros of Ariel killing Ursula:
*It makes Ariel a stronger, more heroic character, not a damsel in distress who needs her man to save her. (This was presumably the main reason for the rewrite.)
*Ariel is the protagonist. She and her father have been the main targets of Ursula's villainy, not Eric.
*Ariel's deal with Ursula leads to Ursula defeating Triton and gaining his power. If Ariel kills her, then Ariel fixes the disastrous mistake she made – someone else doesn't fix it for her.
*In the stage version, Ariel destroys the shell despite the fact that Ursula offers to make her human again if she gives it back to her. In this version, Ariel atones more for her mistake than in any other version, by sacrificing what she thinks is the only power that could reunite her with Eric to save her father and his kingdom
*Ariel saving the day is proof for Triton that she's strong and competent, doesn't need his stifling protection, and deserves to be free to live her own life.
Both variants have their pros and cons, and I'm not quite sure which one I prefer.