How Did The Inquisition Happen?

Cullen Murphy traces how intolerance and persecution took on an institutional life:

The inquisitors shared an outlook of moral certainty. In a world of moral certainty, the unthinkable becomes permissible. The sanctity of private conscience is no longer deemed inviolate. … It all sounds very medieval. But it’s not merely medieval. Scholars may debate whether there truly is such a thing as a “totalitarian” state, and what its characteristics are, but the desire to control the thoughts and behaviour of others – joined to a belief that God or history will render an approving judgement – underlies much of the sad narrative of the past hundred years: the police states, the dirty wars, the ethnic cleansing, the internments, the renditions, the Red Scares, the fatwas, the special prosecutors, the electronic surveillance, the encroachments accomplished in name of national security.

More on the Inquisition as predecessor to today's acceptance of torture here.