You are correct!
The military leader sometimes attributed as the source was Simon IV de Montfort; but Arnaud-Amaury, abbot and head of the Cistercians order and the Papal Legate and spiritual advisor to the Albigensian Crusade, was the more likely source. The date and location are correct, as well. The siege of Beziers, on July 22nd, 1209.
Estimates put the slaughter at Beziers at somewhere between 10,000 and 20,000 men, women and children, most of whom were not Cathars at all (one source puts the estimate of Cathars as about 200 at most). Simon de Montfort was disputably one of the most vicious and bloodthirsty people ever to lead a supposedly religious army. Even the instigator of the crusade, Pope Innocent III, chastised him for killing innocent civilians at one point.
There's a wonderful web site with the salient details and a great bibliography of sources here:
http://xenophongroup.com/montjoie/albigens.htm
Your question, Antagonist?