Europe of the 15th century was locked in the loosening, albeit terrible grips of the Black Plague. Death and decay besieged the landscape. Fear was rife, and the population was riven with it.
In trying times like these, humanity is best served by coming together. But more often than not, we unfortunately choose to tear ourselves asunder. Scapegoating becomes our primary goal. So it was in the 15th century, when the Pope and various countries labeled "heresy" as a corruption to be purged. Witches became the prime target.
In 1486, the Malleus Maleficarum -- "The Hammer of Witches" -- was written and published at the behest of Pope Innocent VIII. The manuscript called for witches to be hunted down and killed, and even contained instructions on how to recognize, torture, and execute them.
Inquisitors sprung up throughout Europe, and, as Carl Sagan wrote in The Demon-Haunted World, the whole draconian enterprise quickly became an "expense account scam":
"All costs of investigation, trial, and execution were borne by the accused or her relatives --right down to the per diems for the private detectives hired to spy on her, wine for her guards, banquets for her judges, the travel expenses of a messenger sent to fetch a more experienced torturer from another city... Then there was a bonus to the members of the tribunal for each witch burned. The convicted witch's remaining property, if any, was divided between Church and State. As this legally and morally sanctioned mass murder and theft became institutionalized, as a vast bureaucracy arose to serve it, attention turned from poor hags and crones to the middle class and well-to-do of both sexes."
In Britain, witch-hunting was so lucrative that it actually fueled livelihoods. Witch-finders, also known as "prickers," received a sizable bounty for each girl or woman they turned over to the church. Because incentives were purely doled out per witch, prickers were often careless with their accusations. One man confessed that he had been the death of 220 women in his career.
In total, between 40,000 and 60,000 witches were executed in Europe between 1450 and 1750. The craze was fueled by inane fear and unquestioned belief, and it only ended when remedied with empirical reason, skepticism, and humanitarianism brought on by the Age of Enlightenment.