I look at it from a D&D alignments perspective. Snow's simply migrated from Lawful Good to Neutral Good or Lawful Neutral at worst. Cora has always been Chatoic Evil. Regina sometimes vacillates between Neutral Evil, Lawful Evil (depending if the laws she's exploiting benefits her) or Chaotic Evil - MAYBE Neutral, bordering on Neutral Good, but only when it comes to Henry - she did try that one time, to her credit, but she's fallen again.
Snow, on her worst day, will never move into the Chaotic or Evil classes. And in the fairytale world of Storybrooke, where the laws of the real world don't really apply, that makes ALL the difference.