This web page on the history of European witchcraft persecution, written by legal scholar Doug Linder from the U. of Missouri, states that many of the distinctive features of European superstitions towards witches - pacts with Satan, flying on poles and brooms, transforming into animals, and the whole smorgasbord of sexual deviancy ect.- came from the persecution of the Cathars, many of whom were coerced under torture into confessing these same crimes. He then says that, following Pope Innocent III's suppression, many Cathars migrated into the Holy Roman Empire- where the bulk of executions occurred during the height of the Witch Trials- from their original home in Southern France and Northern Italy.
While Linder is an accomplished legal and Constitutional scholar, he doesn't as far as I know have an extensive background in Medieval studies and doesn't list his sources for this section of the page. I know that there were many cultural influences leading to the Witch Trials, from the Torah to Pagan Rome to Thomas Aquinas, but Catharism isn't one I had thought of before. Do any experts on this period of history know if this interpretation is commonly accepted? If so, it seems to me that the Cathar heresy is a seriously undervalued event in Western history, given that it would have shaped European notions of Good and Evil from then on.
This is not a commonly accepted interpretation. No real mention of Catharism is made when trying to explain the origins of the witch trials. Inquisitors interrogating Cathars were not looking for evidence of Satanism or black magic, they were looking for evidence of the heresy that the Cathars actually believed in – for example the belief the material world was created by the evil Old Testament God while the spiritual world was created by the good God of the New Testament. Magic and demon-worship did not become a topic of inquisitorial concern until the 1320's, under Pope John XXII. John decreed that black magic was a form of heresy, conflating the two concepts in a way that was relatively novel at the time.
Linder does not mention one of the most commonly cited inspirations for the emergence of the concept of witchcraft: the actual practice and fear of necromancy (black magic) in the Late Middle Ages. Necromancers were typically educated men (often monks, clergy, or courtiers) who wrote, bought, or used grimoires of demonic magic. They did not worship demons, but they did attempt to summon them. Often they used the name of God and other holy figures to compel these demons into service. Their magic was highly ritualistic in nature, involving the drawing of circles and long incantations. Necromancers often went unpunished, but when they were they were tried by conventional secular and ecclesiastical courts rather than by the special inquisitorial tribunals that dealt with the Cathars. It's from these necromancers that many Early Modern ideas about witchcraft evolved.
The sources below are a good starting point for learning more about the evolving concept of demonic magic before the beginning of the witch trials.