Well, disorders said there were a lot of Christians at his/her school (sorry, I forgot disorders' gender, and I'm too tired to go back and try to discern the correct one), and while not all Christians judge this way, many who are anti-Wiccan base it on the biblical passages that say witchcraft is evil, of the Devil, etc. Some people believe the Bible is infallible and every word of it is The Word of God. However, others understand that the Bible was written by man, and it's been translated thousands of times, so what the original said may be quite different, not to mention that the original could have contained errors, and was based on the views of the writers at that time in history. Everyone interprets religious texts (any religion's texts, not just the Bible) differently, and some think it gives them the right to mistreat others, unfortunately.

In addition, the images we've seen of witches in media and fiction have further harmed the reality of some witches. Yes, there are good witches and bad witches. All power--including the powers one gains from religions, political gain, financial gain, etc.--can be used for good or evil. I don't blame fiction for false imagery, because it's fiction, but many people think what is depicted in fiction is reality sometimes. It's better to use non-fiction books and documentaries as sources for getting to know something, but even then, you have to take into the account the views of the person who is doing the depicting. The best way to learn about people is to get to know them.

Wiccans do not worship the Devil, nor do they believe in a devil. They have many gods and goddesses. Even Wiccans believe differently from other Wiccans. I think it's like that with all religions and non-religions. (One atheist's beliefs differ from another atheist's beliefs.) This is why it's better to get to know the person before judging him or her based on something that doesn't describe an entire group.