Same as in canon, Princess Luna becomes Nightmare Moon, and provokes the Equestrian Civil War out of jealousy of her sister. This is also expanded upon in the fic to also be due to her massive unpopularity and her thinking that Celestia no longer loved her any more.
In a smaller example, the pegasi war hero Thunderhide also joins Luna's side after losing faith in the Celestia-Equestrian system, and feeling that his bretheren's sacrifices in the war were in vain.
Trixie starts out the story as The Atoner, who wants to learn true magic from Twilight in order to make up for her boastful and arrogant past. When Twilight turns, she talks Trixie into helping strike at the other Elements of Harmony... then Trixie sees how powerful Twilight is, and performs a full turn herself in order to convince Twilight to keep her.
In the Pony POV Series, Fluttershy suffers a severe Break the Cutie and falls victim to the same corruption as Nightmare Moon, transforming into the Big Bad of the story arc, Princess Gaia/Nightmare Whisper.
The Nameless Passenger turns out to be a version of Twilight who turned into a vengeance crazedNightmare seeking to punish Discord for all eternity via "Groundhog Day" Loop, no matter how many lives she may ruin or erase in the process.
Horseshoes and Hand Grenades has Gentaro do one when Kengo didn't revive him. A being known as Ophiuchus resurrected him and brainwashed him to believe that his friends wanted him to die and left him to the hands of Ryusei Sakuta/Kamen Rider Meteor. Things get very bad.
Happens in the Hunger Games fanfic Some Semblance of Meaning, when Phlox allies herself with Vale, Fen, and Lark, only to attempt to murder them in their sleep. Thankfully, this fails. However, Vale thinks to herself later on that, in spite of her actions, Phlox wasn't really evil; it wasn't the right thing to do, but "this is the Hunger Games. We all do things that we don't want to do—like killing and stealing and betraying—because let's face it: every single one of us wants to escape this sadistic prison the Gamemakers set up for us, to make it home alive. To our friends and families. For that, we would do anything."
Tambryn in the Hunger Games fanfiction Claustrophobia started off as a nice, well-liked character. Then she went off the deep end and tried to crucify her ally and district partner Maxim, who was only thirteen.