You're forgetting this:
So sure, Dominique can be cruel...on all unrelated paths to you changing her....but is that really her true nature? Ynobi-whatever-he-called keeps missing this. Over and over and over again. She is not evil...cruel..perhaps a little (she a Karlsson sister after all)...but still with a moral compass, something that cannot be said of two other sisters.
She is conflicted...which is why Veronica will not lend her any weight towards a vote for her. If she truly was evil and cruel..this conversation (seen by all players) would never have existed and Dominique would have Veronica's vote.
If Dominique's true nature was solidified, but feigned being the opposite, considering how close they are on NOAH and the Soil Restoration projects, and Veronica's protection of Dominique, Veronica would know what that nature is. Instead Veronica accuses Dominique of not having chosen her true nature, she is neither good pretending to be evil, nor evil pretending to be good, she us trying to play both fields, or "have her cake and it to." So Veronica says "If you want my vote, then choose who you really are."
Unfortunately for Dominique, Veronica's assessment is correct, and Dominique knows it, otherwise she would've been able to give Veronica an answer then and there, but all she can do is "glare" in hurt, unable to respond or refute the truth. To choose the Light or Dark path is to completely forsake the other, and Dominique, for some reason, cannot abandon either lifestyle.