I also didn't notice any flinch on Sisko's face after Septimus III. Martok speaks with disrespect about the Cardassian forces (he says himself they were old people and reserve - where is honour in attacking weaker ones?!) and Sisko doesn't care at all. But when the Feds' fleet is destroyed by the Dominion forces - oh, it's such a tragedy I must smash the console with my fist.
Cardassian life is cheap. Clearly. War or no war... Garak said that the Federation has rules in war, which make it more difficult to win. They have rules, but they hardly follow them.