Board Thread:Spoilers!/@comment-6175354-20140113165251/@comment-24892318-20140513023130

Andy and Edward said in an interview before the finale that "evil isn't born, it's made."

That could be referring to Regina; she's not inherently evil. She's practices light magic. But when she's confronted with what she perceives as an unforgivable betrayal, be it by Snow or Emma, she reverts back to dark magic.

I believe she could be at least one of S4's villains.