She's a pretty bad person, if you ask me. She's selfish, unprincipled, greedy, etc. Not that Nucky is a good husband, but he is the protagonist in the story. Margaret has come through huge for him on a few occasions, but she's also undermined him consistently each season, and it's getting worse. I'm thinking they take the story in a direction that makes this a lot more obvious. At this point it's subtle, and maybe she's just a complicated character, but I think they are grooming her to outright oppose our hero.
Maybe I'm wrong...just a theory.