Would she have bought this before taking the apprentice’s heart? Before that moment, I don’t think we’ve seen Regina actually kill. Her heart wasn’t dark even when she threw her mother through the mirror.
But her heart began to darken when she pushed Cora through that mirror and she admits to Rumple that she LIKED the feeling of power and using magic.
Oh, and Rumple indicates at that point that she doesn’t really have a choice in her magic studies; while he was pulling strings all over in the Doctor, he was still giving her the illusion of a choice in the matter. And cuts her off when she’s not willing to go all the way with what he really wants–someone capable of casting the Curse, which requires heart pulling.
Hm. I have no response to this one.