Generally, vampires are seen as evil. They're considered "monsters" in cultural lore. Most vampires give in to their desire for human blood, but the Cullens don't. It's not easy for them, but Carlisle's mission to avoid "evil" acts is focused and strong-willed enough for the Cullens to successfully live without partaking in their natural craving for human blood. Overriding that urge isn't always so easy for Edward when delicious-smelling Bella is around. Edward's conscious desire to not be a monster constantly conflicts with his unconscious urge to drink Bella's blood. Overall, in Twilight, being good or evil is a decision an individual makes, rather that being an uncontrollable fate.