Well, it doesn't have to be, Wicca has very flexible ideology, and practices, one would not be required to be "anti-Christ". Witchcraft is not practiced by all Wiccans and not all Witchcraft practitioners are Wiccans, most are not. Witchcraft is not a religion, Wicca is. Why would one join a religion and believe in something completely different. If one is not Christian, one would be worshiping the god(s) one believes in or has decided not to worship. If one follows a faith path that foes not have the concept of sinthen hoe would one commit a sin.
However, Christians might judge it to be a sin according to Christian beliefs. Again, if one don't follow that faith, how would it apply?