I could definitely see the Sith committing large-scale mass murder, internment camps, and general atrocities on that scale. In general, I find the best way to go about such things is using soft numbers, mentioning the millions left dead, rather than giving a specific number. The only problem here is the the scale and time table. It's only been a week or so since Junction fell to the Sith.
You make this sound like a Michael Bay film, Disciple.