I experience this too. As for good and bad witches, I think, historically, Christian doctrine doesn't even allow for the possibility of good witches. They're bad by definition. What's important is motivation. The need for scapegoats drives and sustains the assumption. In Trask's time (really long before I guess), no one would have any use for the idea of witches, if they weren't evil.
I don't know myself, but Wiccans will tell you about how Christianity demonized what was then a competing religion, in the Dark Ages, I guess.