I don't believe Minfilia has gone evil. However, there are a number of factors that lead me to believe she may have been possessed by Hydaelyn...
1. Crystal chains. In the artwork for the patch, Minfilia is shown chained to the Mothercrystal. Chains typically represent being forced to do something - be it slavery or prison.
2. White clothing. Associated with Light, Hydaelyn's element, and also giving off an air of intrigue and mystery. Simple, but not unexpected of a goddess like Hydaelyn, who doesn't make a big deal out of herself.
3. Closed eyes. In both the artwork and the model, Minfilia's eyes are closed. Even Y'shtola, who is physically blind, keeps hers open. As the eyes are considered the "windows to the soul," I believe this has significance.
Hence, I have a really, really bad feeling about this. Either Hydaelyn's in there with her, or Hydaelyn has subsumed Minfilia's soul and is using her body as a vessel.
... can I beatthe shit out ofsome sense into her? That usually works in these kinds of stories, right?