I think elisi was more referring to the ideas of the soul (equating to goodness), and the slayer, part of a demon, equating to darkness.

And if Buffy hadn't have been the Slayer, then she probably wouldn't have felt so emotionally isolated-"Being the Slayer made me different. It was my fault I stayed that way". A lot of Buffy's issues surround her identity as the Slayer. Her parents might have never gotten divorced, she might have done a lot better in school, and she never would have had a sister relying on her.