This is the storyline I have the hardest time sympathizing with. There are still a few years there, while Snow was growing up, where the main target of Regina’s revenge might have been Cora (maybe this will explain Cora’s whereabouts???). Maybe even Regina was successful in her revenge against Cora somehow (banishment or something, not death) but then realized it didn’t ‘fix’ Daniel’s death. Emptiness waiting to be filled seems to be kind of a theme for Regina (even before curse). Maybe when her revenge didn’t leave her happy, we get to the point in the story where she kills the King and turns her attention to destroying Snow?