Anthy defends Akio, despite how terrible he is to her and everyone around her, because of what he used to be (Dios). She still sees Dios when she looks at him sometimes. How much of what Anthy does is in service to the prince he used to be? How she still, hope against hope, is waiting for Dios to come back?

She betrays Utena and stabs her, and says "you remind me so much of Dios, back when I loved him." And I think that is precisely why she betrays Utena. Anthy can't bear to watch another loved one in her life with lofty ideals rot into a hollow shell of themselves. Dios fell from grace like Lucifer and became Akio. Anthy has seen the same dazzling light in Utena, so better to snuff it out now than to watch her fall too.

And even then, Anthy hesitates to give Akio the sword. Akio sees the hints of rebellion that Utena inspired in her, and is quick to give Anthy the words she longs to hear. "I'm sorry. This is all my fault. I'll try harder next time. I'll be the prince you need me to be."