The concept of duality prevails in the spiritual teachings of the mystic realm.  Finding a path in the darkness does not equate an evil nature.  That in itself is simply a remenant of Judeo-Christian philosophy.  One cannot be a light or "white" witch, without a thorough understanding of what makes a "dark" witch.    To call upon Cerridwen,Kali, or the multitude of other "dark" deities during a time of need, is not evil.  It is knowledge.