Its war, of course its going to bring out the worst in people. The humans in Star Trek are still just that, who will fall back on their instincts and history. Add to that mix Andorians, Tellarite, Zaldans and dozens of other species and you get a wide assortment of people who, though ultimately peace-loving, are unwilling to let all they have and believe in be destroyed.
In "The Siege of AR-558", Quark tells Nog that take away creature comforts and humans can become as ruthless as any Klingon and its true. Most people will fight to protect themselves and/or others, or the beliefs they hold in high regard (even Vulcans understand there is a logical use for force/violence), and that is what Starfleet is expected to do.
Had the Dominion War taken place on TNG, there might have been a greater focus on trying to establish peace, as Picard was first and foremost a diplomat and negotiator. Sisko is a fighter, so it makes sense to focus on the strategy and tactics on DS9.
But that's just my thoughts.