You only hurt the ones you love, but that works both ways. Things said in anger, things done in anger can linger for a lifetime. And there’s no guarantee you can ever take it back.
After being abandoned by her friends, Sunset Shimmer wanders the steets alone, with no hope left, she gives up, only to be saved at the last second. Meanwhile her friends are trying to find her, and apologize for what they've done.
Highschool is a terrible place to be. Tribalism, hierarchy, it can be described as hell on earth, now imagine the my little pony characters stuck in that world. And worse, what if our heroes were Villains?
Twilight just got to the human world(again) and hears the whole situation about the Dazzlings. Adagio sees that she can be a threat and decides to take care of her. But not by getting rid of her, instead she wants Twilight to join them.