"I have a hard time apologizing for what I've done, because no pony ever did so for me."
Starlight finds it hard to make friends after Sunburst gets sent off to magic school. Finding herself feeling isolated, her classmates only make the situation worse. Alone and scared, Starlight begins her gradual decent into her own form of freedom. If she can't fit in, she'll make a world where she won't have to.
Not every villain is evil, and not every evil needs a villain.