Misty is the most interesting, nuanced character to come out of any MLP media, any generation.
Villain heel-turns are common, especially from a villain's not-so-villainous sidekick. But usually they're an afterthought so the audience doesn't get bummed about their fate. Everyone loves the comic relief guy. (This is why Iago gets redeemed in the sequels to Aladdin and why the porcupine guy is seen partying with the ponies at the end of the G4 movie.)
Or they exist to prove the protagonist right. You know--"Heroic Protagonist was gives Sidekick pep talk on how they deserve to be treated better, and later Sidekick has one (1) moment of introspection and switches sides, justifying Protagonist's empathy towards them."
In G5 MLP, the abused sidekick IS the protagonist; she is the one who grows and changes. And in the process she betrays the Mane 5's kindness a lot. But when Misty steals the pages of the alicorn book, we don't think "That terrible pony! Zipp was wrong to trust her!", no, we think "Oh no, Misty! Please, can't you see, no matter what you do it will never be enough for Opaline."
So many times Misty is on the cusp of leaving Opaline and gets reeled back in by Opaline's flattery, intimidation, false sweetness, and scorn. It couldn't work in any other MLP show. It would have been flatout annoying in G4. But in G5 it works.
I cried when Misty finally, decisively, turned against Opaline. Not because that moment justified the Mane 5's faith in Misty (which, in that moment, didn't exist); because it justified Misty's faith in Misty. The belief that, no matter how awful her past upbringing or her past actions, she could rise on the stepping stones of her dead selves to become someone better.
Like a butterfly emerging from a chrysalis.
