I agree that Sith are not raging , homicidal maniacs bent on destroying all in their path (at least not the ones who win an empire), that kind of behavior is counterproductive to running a stable empire, a certain measured amount of fear might be used to gain the desired cooperation of the population, but it is equally important to praise and reward desirable behavior in subordinates. The bloodthirsty killer who slaughters for no reason seldom achieves any significant amount of power beyond having people run in fear at his approach. Power is the goal of a Sith, not slaughter for the sake of slaughter. Although razing one town that refuses to surrender to the ground will make it easier for you to convince the next town to surrender, but if you raze every town to the ground you will not be obtaining as much power as you could. I look at the example of Genghis Khan when I imagine how a Sith might manage the conquest of an empire.