He's just not a cackling bad guy. He quietly but firmly gives the command and then observes the barrage.
He is much like Tarkin in that he can give awful orders while being calm and collected. It is the banality of evil that authoritarian systems create.
From a narrative point of view it highlights that the decisions being made are bureaucratic. The Empire is divesting itself of clones for partially financial reasons. Rampart is as much bureaucrat as he is military. Studied indifference is the trait of the bureaucrat. Impersonal and without emotion. Most military men are ruled by a code of honor, loyalty above all else. The Jedi and Clones personify honor and loyalty. To one another and to the cause. They disobey orders to leave no man behind. To do the right thing. But Rampart isn't like the military men of the Republic. He is a new breed taking their place.
That he can give such an order with no emotion demonstrates that the system is paramount, it is above human feeling and the individual is unimportant. To him destroying the city and killing that team is no different than giving the order to purchase 1000 new pairs of boots.
It is also emblematic of the Empire as a whole. Individuals, whole populations and even planets are not important. Emotions, feelings, are all irrelevant to the supremacy of the State and the will of the Emperor.
Or he's really just a cool customer. I used to give commands to fire when a tank commander not unlike what he did.
A firm "fire on my command... fire" will do. No shouting, no emphasis. Just a routine command.