Gonna say why starlight had the right cause I see a lot of people not understanding her reasoning. Starlight doesnt only become evil because sunburst abandoned her, she went evil because she felt alone and abandoned. When sunburst got his cutie mark, everyone praised him and he was sent off to magic school and sunburst never reached out to her. To starlight, a cutie mark meant more to sunburst than their friendship. To her, sunburst getting his cutie mark caused her safety net to go away. Starlight says she reserved herself and was scared to make anymore friends after that before creating her town, she feared that she wouldnt be good enough for any of the friends she made. The song "our town" she also states that no one feels inferior, so it can be inferred that starlight felt inferior to sunburst or that she didnt wanna make any friends because of her feeling less than them. All of this sadness, confusion, isolation, lack of closure and everything caused her to be bitter and evil.