Tina,
WHy is there evil?
What motivates those who perpetrate it?
How do they reconcile their actions with a self-image that does not embrace evil?
At least half the problem with discussing a complex issue like "evil" is to actually define the term.
Once you have hammered out a meaningful definition of "evil", you might find that the definition itself answers the question pretty good. And then, you probably discover that the definition was inadequate, or that other people operate with other definitions.
The question also seems to assume there is some objective truth about what behaviour is evil. Many religionists, for example, will argue that sexual relationships outside marriage is evil. Others will disagree.
As I see it, the existence of the concept "evil" is not very hard to explain. As Seeker said, evil comes froms selfishness. But it is far from an adequate description. Most of what we do is motivated by a degree of selfishness. And in most situations of life, we grasp for a good that is in limited supply. Competing successfully for your goal may deny it for another person who wants and needs it just as much. When is struggle for resources necessary for survival, and when is it "evil"? Not necessarily easy questions.
- Jan
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Faith, n. Belief without evidence in what is told by one who speaks without knowledge, of things without parallel. [Ambrose Bierce, The Devil´s Dictionary, 1911]