As a working definition, evil is the will to intentional badness. You can't be an evil person without consciously and intentionally willing something bad upon someone else. There is also no such thing as an intrinsically or objectively bad thing or event; something is only evil by intentionality, by subjectivity. Another distinction should be made between an evil person and evil actions: the former is a person whose intentionality has become such that he constantly wills badness on others, whereas anyone can commit evil actions intermittently.
What causes someone to be evil? We often speak about this like people choose evil just for the heck of it, or are evil because they differ ideologically or culturally than us. But I think what motivates evil is accumulated perceived injustices in a person's life which the person displaces onto others in a decontextualized way. "The world screwed me over, and so I'll screw the world over back," is the motto for evil. One commits evil as a way of punishing others for the unfairness that the self perceives has been done to it, not just from the particular other but also additional instances of injustice done by others as well. Evil often involves overpaying the other person in badness given this displacement, and this is one of the big problems of evil: it repays too much badness on a single person at any given time. Hence, perhaps, the "blindness" involved with evil. Think about the people who grow up in bad homes or neighborhoods. They're much more likely to commit evil actions because they've had more accumulated injustices thrown their way.
Another way of saying this is that evil would be impossible without a standard of justice. If this is true, then all evil has an element of goodness in it, and in a real sense goodness is a main ingredient in motivating evil. This goodness (justice) is twisted or misapplied, but still, without a sense of justice there could be no evil.