So is she a "grey" character or not "all that stable" aka "something's wrong with her head"? Her backstory is a pathetic joke. I'm pretty sure at least 90 % of all the people in the world had to suffer more than Starlight Glimmer and less than 1 % of us become maniacs like Starlight Glimmer just because of some bad experiences. And no, I don't think there is more to it. That's it. She lost her only friend and that's why she became a psychopath.
That's why I don't feel sympathy for her. Her motivation for being evil sucks and everyone forgave her way too quickly.
Sunset Shimmer was just extremely selfish and regrets it now. Not really a good motivation either but I at least kind of get it. Starlight Glimmer's motivation on the other hand is just not believable or understandable to me. To me she came off as a weak crybaby who punished the world for her problems to be honest with you.