The overall idea of types of magic, like Black magic, or White magic, largely stems from applying Ceremonial uses of magic practice to Witchcraft.
The dark side of nature and the light side may naturally find balance in the world.
The idea that black magic is evil and white as good, with grey being somewhere in between, stems from assumptive mainstream ideas about magic.
Using dark power to harm another can be seen as going against a Witchcraft code.
The Yin and Yang of Oriental systems is much more applicable to the Green Witch: dark and light in equal measure, finding balance in the same space together.
We have the Lord of the Shadows (another aspect of the God), giving rest and caring for the departed, as well as the Dark Mother (another aspect of the Lady), leading us from this world and into the next.
Context and perspective need to be in place when trying to describe these things, which helps us to see the dark side of magic is not evil at all.