Board Thread:General Discussion/@comment-4962906-20151111090025/@comment-26715865-20151111120028

She killed Vortigan, as revenge for burning her villiage, it was an act of hatred. She had the power from the Holy Grail and killing Vortigan made her the Dark One. I also hope their going to explain better, but for now this is the reason she became the dark one. Of course we can make theories.