Im pretty sure is ventress the trailer realy shows its her. however i dont think she is the vilan here. Most likely its a job that someone paid her to do. I dont think she has enough motive to do this on their own specialy since she bercome a bounty hunter, and helped obi wan against Maul / savage.
I dont think she hates anakin or obi wan enough as she hates dooku imo. In fact the last aperences she seemed practicaly hate free wiling to start a new life. Maybe she is working with the jedi secretly to uncover some plot who knows. For her being turn so bad so fast it better be a dam good reason.
And im confident maybe Filani is behind something too.


http://www.youtube.com/watch?v=SFQRoVlePvg

in the minute 3:05 cleary ventress there. But im highly doubt that she is the bad girl here.