A few things:



1: You should be more specific. Leaving it up to interpretation just brings forth unfortunate implications.



2: Why was Celestia villified then? That statue implies that Nightmare set herself up as the hero and her sister as the villain just to gain her subjects love.



3: Again, the problem is that Celestia isn't evil. She's doesn't make a spectacle of herself and declare herself the new ruler like Nightmare did, but just came back from her banishment after her sister staged a coup. So no matter the situation the Elements STILL shouldn't see her as evil! This just comes off as forcefully brainwashing her instead of trying to reason with her, despite her wanting nothing but her sister back. And those evil grins of theirs don't help matters either.



4: The reason the Elements worked on Nightmare is because she was corrupted and had to be banished for the sake of Equestria, there was absolutely nothing wrong with Celestia, so she still isn't evil and therefore her needing to be stopped is only because bit Nightmare's actions.



5: Show don't tell, as far as the picture series show, Nightmare corrupted the Elements, then siced the dark six on Celestia. You want this kind of story to work? Don't just have it end with her being converted. Either:



Have that cult get to her first and attempt to get her to take down the Dark Six. At first she defeats them But when she witnesses Nightmare's actual legit care for her subjects, she ends up relinquishing the Elements or something...



-OR-



Don't try to sell it as them being heroes, this is only a "bad guy wins" timeline and the dark six are anti-heroes at best. While Nightmare isn't a tryrant but more manipulative ruler, making sure any and all threats to her rule are taken care of before they become a problem.



6: You can't truly justify Celestia being converted as a heroic act, because she really has no reason for us veiwers to see her as the villain. Saying her daylight would end Equestria is a stretch the elements once ended this chords chaotic rain and immediately converted everything back to normal and a proper living environment.