I don't know, their actions are no better or worse.
I suppose you can say that Starlight had good intentions, but that is marred by the way she treated her villagers, basically enslaving, mistreating, abusing and lying to them and even willing to go as far as attack them later on. She acted mor like a selfish dictator then the well-meaning extremist she was made out to be. Tempest did have a tragic backstory, but she was more then willing to let potentially thousends suffer and maybe even die if you want to go that far for herself. It's entirely possible that she wouldn't have betrayed Storm King had he just restored her horn. And Discord lacked any sort of backstory or any sort of potential redeeming moment prior to being given a second chance. He was just evil for the fun of it.
With the trio, you can argue that Chrysalis simply saw herself as doing her job by feeding her people only to be held back by her own traditionalism while Tirek wanted to impress her father and Cozy completely misunderstanding what friendship meant. Sure, it doesn't immedietly make them tragic or guerenteed to change, but I don't think their any worse then what the reformed villains used to be.
Again, it seems to all come down to luck weather or not someone gets the help he or she needs to change rather then anything else and that just rubbs me the wrong way, especially when the heroes are portrayed as morally good role models by the end of the series.