You do not need to side with Katherine in the quest in order to romance her in the end. I chose to let them live and be judged by the law. In the end she mentions that the murderous servant got the noose and Madeline Von Grolle now lives in a house some where but is under guard. Katherine is not happy with this but it still doesn't dissuade her from jumping your bones in the end. Also as a note I did somewhat look for Madeline Von Grolle after the end of the game and couldn't find her.
