I think she's more of an antagonist than a pure villain in the event. I just get the feeling that her role in IvX might be the mix of Tony in Civil War and Wanda in HoM. And I believe actually she's extremely mentally unstable so far and eventually gonna truly lose it. Poor Emma.
The true Emma is Morrison's Emma, canon. In my opinion, the villainous Emma was just cartoonish and would be stale now. I think what makes Emma unique and standout from other characters is that she has a lot of factor leading her into the dark side but she's still not falling apart, which can be represented perfectly only when she's with X-Men. She's going through so many traumatic things to be what she wants to be, that would be tragic and meaningless if she finally went full circle.
And Emma's way of becoming an X-Man was utterly compelling for me. She had never been an arch-enemy of X-Men; there were many battles between the junior X-Men, the New Mutants, and Emma's own trained Hellion, with some New Mutants eventually switching teams as what began as a bitter enmity gradually became an un-serious "rivalry". Professor Xavier even believed that the purpose of Emma's school was instructional in the same manner as his; she had wanted to be a teacher, after all. That's why Xavier tried to console Emma after she found out the death of the Hellion and agreed to the Massachusetts Academy, then renamed the school for gifted youngsters, being used to train a new generation of mutants, as a gesture of faith; and Xavier let Emma be the tutor and headmistress of the school. When Banshee doubt if Emma was a good choice to be those kids' teacher, Xavier was the one who vouched for her in UXM 318. In Gen X #75, Emma talked with Xavier on phone and left in a helicopter; then we knew that she was teaching in the Genosha after that. Xavier helped to cover up for her from the investigation of Adrienne's death. And in New X-Men #116, Jean Grey said to rescuers "Emma! She's one of us. X-Men!" All of those can indicate that Emma joining the X-Men was meticulously and originally planned, in a convincing way.
In dog days, all we need is Frost.