Catharism
A mistake by the defenders of Beziers let thousands of attackers in. Arnauld Amaury made the famous quote "Kill them all, God knows his own"  on being asked how to tell who were Cathars during the assault. Everyone in the town was killed, some while taking refuge in the church.  It is guessed that 20,000 were killed, many of whom were Catholics and not Cathars at all.  The crusade became known as the Albigensian crusade after the town of Albi.  It was to wipe out the Cathars almost entirely over forty or so years.  The Crusaders wanted to go home, but were ordered by the Pope to continue until the whole South of France was controlled and all Cathars were dead. In 1210, they attacked the fortress at Minerv and built "the first great bonfire of heretics" - beginning the practice of burning at the stake that would continue in the Inquisition of the Counter-Reformation. It is interesting to note that at the siege of Montsegur when the fires were lit the Cathars ran down the hill and threw themselves on, as their beliefs were very strong.