Sometimes the beliefs of hyper-rational men are subject to long held prejudice. Arthur Koestler would have liked to have been thought of as a man who followed the evidence where it led even when it seemed to stray into avenues of unreason according to scientific rationalists. What though are we to make of this commonplace observation about the depredations of religious persecution.
This may sound like a psychological paradox. Yet I think most historians would agree that the part played by impulses of selfish, individual aggression in the holocausts of history was small; first and foremost, the slaughter was meant as an offering to the gods, to king and country, or the future happiness of mankind. The crimes of a Caligula shrink to insignificance compared to the havoc wrought by Torquemada. The number of victims of robbers, highwaymen, rapers, gangsters and other criminals at any period of history is negligible compared to the massive numbers of those cheerfully slain in the name of the true religion, just policy, or correct ideology. Heretics were tortured and burnt not in anger but in sorrow, for the good of their immortal souls.(from The Ghost in the Machine)
Torquemada would be generally accepted to be the most assiduous Inquisitor. The Inquisition was continued after his death by Isabella and it is interesting that the Queen’s secretary estimated that 2,000 were executed during her reign.
There are various estimates of the number of victims of the Spanish Inquisition during Torquemada's reign as Grand Inquisitor. Hernando del Pulgar, Queen Isabella's secretary, wrote that 2,000 executions took place throughout the entirety of her reign, which extended well beyond Torquemada's death.(from Torquemada)
That would be a period of 26 years. Now considering the number of capital crimes in that era I would say this was not anything like the slaughter that Koestler assumes.