From what I've gleaned through the Fandom attributing to her fall to the Dark Side of the Force, Shimmer once had friends similar to those Twilight has, however an unspecified accident happened that resulted in the tragic deaths of everyone save her. Because of this, Sunset blames Celestia for not being able to protect them all (attributed by the simple fact that of all the times that something of great magnitude was befalling Equestria: Nightmare Moons' return, Discord unleashed, and the events leading up to the Changeling Invasion: Celestia did absolutely next-to-nothing to intervene or stop them from happening altogether. Only appearing after Nightmare was purified, sending letters to Twilight instead of appearing herself when her friends were discorded, and failing to listen to her apprentice when she claimed that there was something wrong with Cadence after she was replaced by a Changeling; despite the fact that twilight personally KNEW Cadence since childhood, even when she could have intervened it was already too late for Celestia to do anything of any degree... Same could be said for Luna, where was SHE during the commotion of the Invasion?)



I'd personally place Sunset somewhere along the lines of Bengel Morr: a Nautolan Dark Jedi that was originally an apprentice of Orgus Din, a member of the Jedi Council during time of the Old Republic. On the last day of the Great Hyperspace War, the Sith Lords Darth Malgus and Angral led a full-scale attack on Coruscant: Killing the Supreme Chancellor, holding the Senate hostage, crippling the megacity, and devastating the Jedi Temple. It was only the timely intervention of the Treaty of Coruscant that put an end to the war and all of the madness, although the damage was done. Orgus could not find his padawan and assumed he died in the carnage, but he survived. Bengel survived the madness and was vocal about the conflict, where in the moments before the Treaty was signed, he saw Orgus surrender to the Sith. From this moment, Bengel began to think that the Jedi Order itself was based on weak foundations and sought to supplant it with stronger Jedi by sacrificing the weak, traits similar to the Sith Order. Banding together the Flesh Raiders of Tython, the ancestral birthplace of the Jedi, Bengel sought to bring his vision of a stronger Order, by killing his old master.