Before someone would say that’s impossible because of the magic of friendship and harmony. I have an example of a hero turning evil. If anyone remembers Butch Hartman’ Danny Phantom, the episode Ultimate Enemy where Danny meets the most evil ghost he ever fought, his future self and learn of his origins on how he became the destroy of both the Ghost Zone and Earth. So what if Twilight not only loses her friends, Shining Armor, Cadance because of her magic and Flurry Heart hated her for lose of her family while trying to stop Tirek, Chrysalis and Cozy Glow as they get their revenge of them. Discord can’t undo what happen and Starswirl’s Timespell can’t take her back to save them. She abandon Equestria and begged her worst enemies to remove the Element of Magic from her, only to it become sentient and absorb her decease friends elements to become Dark Harmony (Reason for the name: Without Twilight’s emotions to control it, it attacks everything that once had disharmony) and destroyed Twilight Sparkle and unleash her wrath.