One of the key themes that you will find in fantasy stories is the concept of good versus evil Sometimes there is a clear deliniation between the two - often represented by widely accepted stereotypes (witches are bad  knights are good) but more and more the lines are being blurred as we accept that often there is a 'dark side' in all of us  or we can 'fall into darkness (Darth Vader) or be redeemed (Malfoy)