Is witchcraft a sin

To the Christian religion yes, not Wicca though.

As this question was asked in the Paganism category, one Pagan answer is "most certainly not". The concept of "sin" is not really one that Pagans in general adhear to.

Witch craft - the craft or practice of a witch often known as magic is the manipulation of naturally occurring forces by the hand and will of a (hopefully) ethical practitioner. There is nothing "sinful" in what we do.