You have a point there. She did not have much time to think in that moment and Buffy tends to react quickly on the gone-evil-condition, since she has witnessed it so often. Also, it's always pretty cool when Faith and Buffy fight, so I can see how Vaughan would not want to let that opportunity pass.
I guess, I'll wait and see how things turn out in #9

Thanks for commenting, makes me happy to have some people around here :)