A while back, IDW released a storyline known as Ponies of Dark Water, where the Mane Six themselves became evil.

I can't help but wonder, why aren't there any stories centered around our mane cast's sudden fall from grace? Surly there's some potential to be found here?

I considered writing an AU fic involving Twilight turning evil, but I never wrote it since it would've been too time-consuming (plus, I wasn't sure how to end it without it feeling like too much of a cop-out).

If anyone's curious, the idea itself would be an Alternate Ending to the Royal Wedding episode, where Spike was accidentally killed by the debris blasted out from Shining and Cadance's changeling-cleansing spell. Twilight wasn't able to get him to the hospital in time, leaving her a devastated wreck by the time her friends and family arrived to the hospital. The loss of her adopted son/brother, combined with the betrayal she felt from everyone at the wedding rehearsal, causes Twilight to completely snap and cut ties with everyone she loved.
She would almost kill Shining when he tries to console her, using her magic to throw him through several walls of the hospital and breaking several bones. She completely shuns her friends as she leaves the hospital, not even looking at them (just as they did to her at the rehearsal). When Celestia later finds her and tries to make up for what happened, Twilight slaps her after learning Celestia lied to her parents about where she was before the wedding. After quitting her rank and escaping Canterlot, Twilight would be collecting her things at the library by the time her friends try to talk some sense into her. This would lead to Twilight absolutely destroying them all verbally for being responsible for Spike's death, and threatening to do the same to their loved ones if they ever contacted her again.
Of course, the idea is that all of those scenes would be flashbacks during the main story, which is Twilight traveling as a fugitive towards Chrysalis' main hive to kill her and all the changelings in revenge. While the Princesses and her friends desperately try to look for her, Twilight tries to maintain her sanity while constantly envisioning Spike trying to stop her and talk her out of her anger. And even though her friends were all distraught about what Twilight was turning into, they want to try and find her in order to get her to use the time-travel spell from Lesson Zero; they hope that if they find her in under a week, one of them could go back and stop Chrysalis before the wedding ever happened, thus saving Spike and their friendship with Twilight.

I honestly doubt I'll EVER write this due to how much I'm writing already, but I want the idea out there in case it inspires anyone. If anything, it's a better Royal Wedding AU than just "Twilight lashes out at her friends and Celestia and acts like an OOC bitch".