I think one of the messages of the show is that there isn’t pure evil or pure goodness. I’m surely not defending what Regina has done so far, but we need to learn of the real reason behind it, yet. Yes, she’s evil, but she’s also desperate and lonely, not to mention her affection for Henry (not the healthiest, but it’s there). Pure evil wouldn’t feel anything of the sort. Pure evil doesn’t need a reason for its acts and just does it for the pleasure of doing it. We haven’t seen any of the characters do that so far. They did horrible things, but they always did them because they had a reason. It wasn’t always understandable to us – and it shouldn’t – but they were driven to do them by their emotions, something true evil doesn’t really have.
I like to use Lord Voldemort as an example of pure evil. The guy truly didn’t have any morals, didn’t care for anybody or anything but himself. He did the things he did because he could do it and because he found pleasure in them (and because he thought he was right).