Witchcraft in the Middle Ages

Recently I read a book detailing the history of the treatment of witchcraft in the Middle Ages, starting with around Roman times and going through the 15th century. It was generally good, though I found the author a bit too credulous regarding some of the accusations made against witches/heretics in the later middle ages. I'm probably going to be doing several posts on this book, but I promise nothing.

One of the things I found most interesting was the shift in the legal treatment of witchcraft. Under Roman law, witchcraft was punishable based on the underlying offense. So, if you used witchcraft to commit arson, you would be punished as if you had committed arson. But, as the secular power of the Church grew, and the identification of witchcraft with Satan grew, the punishments grew more severe. The reasoning was simple -- the government derived its power from God, witches aligned themselves with Satan, therefore witchcraft was treason.

Not really bad reasoning, if you buy the premises (and, for the record, I don't -- I like liberal governments). The problem was in the standard of proof, most especially the admissibility of evidence obtained through torture. I don't like torture, and it should be clear now, and even should have been clear then, that it is not a reliable method for obtaining evidence (moral objections to it aside).