In TNG the Borg were like bees. In Voyager they were more like Buggers.
The Queen didn't seem to have any real motive for her actions in Dark Frontier. Her actions were more response to plot demand than a logical consequence of established Borg precedents. The writers wanted Seven to be confronted with her old Borg life and tortured by its ethical implications and the queen behaved accordingly. Stage 1: Kidnap 7. Stage 2: ? Stage 3: Galactic domination.