I suppose it depends on how you look at Cora. Her backstory actually made me feel bad for her, and that once she did feel love, but she herself threw it away for power. She always had a choice to turn away from power for love, but she chose the latter.
Evil is such a dangerous word to be just thrown around lightly. Regina originally seemed to be a nice girl, but she ultimately became the thing that she hated. With the parents she had, there was probably at least some degree of genetic predisposition to the thoughts which she had.
We all are capable of good and evil. Circumstances, personal choices, need ,temptation a lot of things determine the courses of our lives.
Cora's lust for power is what corrupted her so much. Even back in Storybrooke, where she entered mayor's office with Regina, she immediately went and sat on Mayor's chair, depicting her inner wish to control the power wielded by her daughter by controlling her.
Cora had it in her. Being publicly humiliated by some spoiled princess could have exhacerbated her discomfort with her poor condition, but the extent of her evil actions went far beyond that. Besides, Rumple had to bring people from other lands to manipulate Regina into becoming evil, while Cora basically bought herself into it. Heart or not, she had some evil in her to begin with, which would eventually grow out of control.
Cora is not evil, she really wanted the best for her daughter, but she had no feelings because she lacked her heart, so she could just think with her brain but feel nothing or empathize with anyone's feelings, that's why she did all what she did.
I think that, once she had Regina, if she would have still her heart inside her, she could have been a good mother and person, as she's not really an evil person, she just... does things her way, without be able to feel or know how would other people think.
After watching The Miller's Daughter again, I've come to the conclusion that yes, Cora was really evil. She did all the wrong choices, but she did them deliberately. She learned she was fifth in line so she decided to go away with Rumple, who was clearly more powerful than her to-be husband. Perhaps she somewhat loved him too, but that's the reason. Then King Xavier made her change her mind offering her the power she wanted and she swinged back because power was more important to her. It's true, she did many evil deeds while heartless, but the choice to take off her heart to have power over love was ultimately her own.
She probably changed her mind only in her dying moments after realising that she wasted a whole life going after something that wasn't worth the sacrifice, so she told Rumple first and Regina then "this would have been enough", but I do really think it was a very last moment realisation.