As I have stated before, Wicca has absolutely nothing to do with the devil. It is a religion based around feminine strength and nature. That doesn't sound like Satan worship to me. After all, you believe in God if you believe in Satan. Wiccans don't believe that there is any one evil being such as the devil.
Nothing is either pure or corrupt.
In other words, whatever you send out, you get back, only three times more potent. The girls who tried to kill their fellow Wiccan were punished, one of them mentally insane while the other two lost their powers ultimately while the one they tried to murder became even more magically powerful. I doubt that Miss Rowling knows much about the Craft, if any at all. Personally, I'm not a religious person and I'm happy with my decision. Maybe these people are angry that a non-virgin woman with her bastard child is now the third richest person in Britain.