As the head of the Cullen family, Carlisle presents a center of calm. He's always there when someone (often Bella), needs a doctor, and his belief in the rightness of the non-violent vampire life he has chosen for himself and for his family appears to be unshakable. When he suddenly sees his family confronted with an evil vampire army, Bella notices the "stress on his face" (17.29). Carlisle also spares the newborn vampire Bree's life when she surrenders, because he believes she hasn't been taught to be good.
For more info on Carlisle, check out his "Character Analyses" in Twilight and New Moon.