One of the facotrs of redemption is remorse. Sorry to say it, but I have yet to see Regina actually feel remorse for what she has done. She killed Graham, seperated Hansel and Gretal from their father, locked Belle up, and had repeatedly gone after Prince Charming and Snow White. Pretty sure that in their world, she would end up in prison or executed for her actions.
The whole Charming family has stood up for her, protected her when she needed it–not once did she thank Emma, Snow White, and Prince Charming for their actions in rescuing her. Instead, she treats them with scorn and contempt, while still expecting them to invite her into their lives.
What she has done was nice, but all she did was repay them for what they had done. If it had not been for them, her soul would have been sucked.