Personally, I don't mind having Starlight being a redeemed character. Lot of people think her reasoning for being evil is too childish, but hey! It's a kids' show, plus, Starlight was a child at the time and children are impressionable, and from what we've seen in the beginning of the season 6 premiere, it was very traumatic for her at the time. Sunburst must've been her one and only friend. Also, it was probably from then on that Starburst started fantasizing of a world where all ponies don't have to be superior nor inferior, with all the same Cutiemark. Then making it her ideal world which became her dream.



What I'm saying is, the tiniest things that happen in your childhood, could infect your future. Like how Rainbow Dash's Rainboom led to the Mane Six getting their Cutiemarks which led them to be who they are now.



My Headcannon is that Starlight Glimmer is actually an orphan and Sunburst was the only one who reached out to her. Thus, making her turn evil more understanding.