I love Starlight Glimmer as a villain. What she did was pretty horrid and may not have been Eternal Night, Eternal Chaos or even Slavery but taking away what made a pony special for her ridiculous philosophy. Not to mention stopping the Sonic Rainboom more than once so the future gets all screwed up (that was unintentional though)
Her reformation was ok, but very rushed and disjointed. And her backstory was terrible - her motivation for starting Our Town was that her friend Sunburst left her to go to Celestia's school? Seriously? And we never saw how Starlight earned her cutie mark. Was she also sent of to Tia's school? And if she was, why didn't she attempt to make contact with Sunburst?
Her backstory would have been better if she was a pony Dictator or something. I would have even settled for her as a radical SJW