Aaron's recent entry sums up many of my thoughts on evil. My experiences reading about and talking with people who (in my opinion) clearly commit harmful acts have shown me that, in general, everyone believes they're doing good. What may appear at first to be an obvious act of evil is often outweighed in the perpetrator's mind by a greater good that may not be apparent to others. Instances of truly intentional harm probably take place commonly at a small scale, but i suspect that large-scale, premeditated, cognizant harm happens only in comic books. (And even so, many comic book stories explore the idea that evil is not so simple.)
As for Aaron's concluding statement that good intentions are no defence, i only partly agree. I think it's reasonable for intentionality to affect the way we treat or punish people who commit crimes. Human behaviour toward other people (and the legal system, by extension) is based on a predictive model of how people will act, and knowledge of intentions affects the predictive model. Nonetheless, good intentions will never un-rob, un-rape, or un-murder anyone, and an appeal to good intentions does not allow one to escape responsibility for evaluating the consequences of actions.