The Wikipedia has a fair amount of information on the Cathars that seems fairly well written. I noted in particular a description of habits and beliefs that upset the Catholic organization:

The slaying of life was abhorrent to the Cathars, just as was the senseless copulation that produced enslavement in matter. Consequently, abstention from all animal food except fish was enjoined of the Perfecti. (The Perfecti apparently avoided eating anything considered to be a by-product of sexual reproduction, including cheese, eggs, milk and butter.) War and capital punishment were also absolutely condemned, an abnormality in the medieval age.

Such teachings, both theological and practical, brought upon the Cathars firm condemnation from the religious authorities whose social order they threatened.

It is amazing to take a careful look at life under Count of Toulouse Raymond VI as he was the ruler in an area with a fairly civilized and yet secular and prospering community. Other sites paint a compelling portrait:

Chivalry and poetry, art and literature, and business and commerce thrived, the land was at peace, the people prospered; Jew, Cathar, Waldensian, and Catholic lived and worked side by side in an area free of religious persecution and dominated by religious tolerance. People were judged on their own merits, not by the religious beliefs to which they subscribed. Secular offices were distributed on the basis of merit, not religion, and powerful offices in the state were open to Jew, Cathar, Waldensian, and Catholic alike.

This was the time and place of the Troubadour poets, influenced by the Sufi poets to the south as well as the Kabbalists. They sang and spoke of the idealized female and unattainable love, and recognized women as spiritually equal; perhaps even as divine. In fact, women even could become a Perfecti!

Things went south (pun not intended) from increasing pressure from the Pope who became outraged at the Cathars prosperity. He demanded they submit to the Catholic establishment’s authority and he sent representatives to negotiate their surrender. Sadly, one of the representatives was murdered while visiting Raymond. This led to excommunication of Raymond and eventually a Papal decree that the land and wealth of the region was fair game to Catholics who wanted to take it by force for themselves. From the Wikipedia again:

The crusader army came under the command, both spiritual and military, of the papal legate Arnaud-Amaury, Abbot of Cîteaux. In the first significant engagement of the war, the town of Béziers was taken on 22 July 1209. Arnaud, the Cistercian abbot-commander is said to have been asked how to tell Cathar from Catholic. His reply, recorded by a fellow Cistercian, was “Caedite eos. Novit enim Dominus qui sunt eis.” — “Kill them all, the Lord will recognise His ownâ€?. The doors of the church of St Mary Magdalene were broken down and the occupants slaughtered. 7,000 people died there including women and children. Elsewhere in the town many more thousands were mutilated and killed. Prisoners were blinded, dragged behind horses, and used for target practice. The town was razed. Arnaud, the abbot-commander, wrote to his master, the Pope: “Today your Holiness, twenty thousand citizens were put to the sword, regardless of rank, age, or sex.â€? The population of Béziers was then probably no more than 15,000 but with local refugees seeking shelter within the city walls, the number claimed, 20,000, is possible.

Raymond had anticipated the invasion to a degree and apparently tried to negotiate, but his efforts failed to stop the crusaders from taking Béziers. The town apparently had only a few hundred Cathars, but the decision to stand together meant the whole population was massacred. This crusade continued for more than twenty years. Raymond tried also to comply with the demands of the Pope and crusaders over time to achieve their forgiveness, such as turning-over his seven best strongholds and his mercenaries or removing all Jews from positions of power, but the Catholic authority simply tortured him with his own desire and acquiesence. Eventually, against some resistance, hundreds of thousands of soldiers were sent by the Pope to “convert” or kill the people of Languedoc, and the poetic and peaceful Cathars were completely destroyed.