I believe in pure good and pure evil - but I don't believe they're concepts we can apply to humans. I'm not a religious person, but I have the desire to believe in some sort of higher power that is truly good, or truly evil - I imagine it would be incredibly difficult for most people of faith to accept that any higher being could be flawed, such are the concepts of good and evil.
And they're good markers - most of us can tell when we've done something "good" or something "bad", we generally have good morals and ethics and can recognise where the line lay. The only difference being, the line moves for every person/situation, and so we can't have strict barriers.
Black, White and Shades of Grey for me, please.