The source of their powers
It kind of upends the whole misunderstood-witchcraft-thing if they get their powers from bloodthirsty pagan gods. I mean, there's no real difference between evil pagan gods/goddesses and the Christian devil.
It'd make more sense, to me at least, that witches are born with a portion of a spiritual nexus (like the All in Charmed) that only gods/goddesses have access to, giving them magical abilities. Interestingly, the first witch in that series is also immortal.