The full quote from the film
I too was an Inquisitor, but in the early days, when the Inquisition strove to guide, not to punish.
AFAIK the film is a good adaptation of the book, and Umberto Eco's work is usually well documented.
A small but important correction: given the chronology and geography of Eco's book, William is talking about the Roman Inquisition, not the Spanish Inquisition, which won't exist for another century and a half.