An inquisitor was an official (usually with judicial or investigative functions) in an inquisition – an organization or program intended to eliminate heresy and other things contrary to the doctrine or teachings of the Catholic faith. Literally, an inquisitor is one who "searches out" or "inquires" (Latininquirere < quaerere, 'to seek').
In some cases, inquisitors sought out the social networks that people used to spread heresy.
There were multiple national inquisitions with different approaches and targets.
Controversies
In the Albigensian Crusade a second-hand story arose that inquisitor and general Arnaud Amalric at the storming of Béziers advocated general slaughter, saying “Kill them. For God knows who are his.”[1] Amalric's own report to the Pope was that his troops jumped the gun and took over the town violently before he was aware.