I think that even her change of heart was a really sloppy transition.

Brahne: Durr Imma kill Garnet now that I have her Eidolons
Beatrix: What?
Brahne: You heard me! Shut up and go find the last jewel!
Beatrix: Yes, your majesty...

Five minutes later:

Beatrix: I'll kill all enemies of Alexandria! Never step on to this land again. Your powers are useless against me.
*sees Garnet*
Beatrix: Oh my god, she was really going to kill Garnet. Oh yeah and sorry for the genocide in Burmecia as well.