Obviously the official story was that there were Cathars, who had to be destroyed because they were evil heretics, and so they were. But later historians said maybe this was a lot to do with northern Knights wanting southern land/gold, extension of state power, etc. But when Cathars occasionally come up on this sub, I've seen a new historical view, which is that the Cathars didn't really exist, with their Perfecti and all that, the Church just thought they did. But presumably the idea didn't just come up out of nowhere.
So what really was happening in southern French religion?
It's not that Cathars did not exist, it's that a lot of what we know about their beliefs comes to us through writings of their enemies.
It is certainly not in doubt that in the late twelfth century and early thirteenth century there were many groups of celibate laity in both communal Italy and in the Languedoc and Toulousain in France, who called themselves 'good men' and 'good women'. Their enemies called them Cathars, especially in Italy. Lay religiosity is not uncommon in the central middle ages. Before the Cathars we have the Pataria in Milan in the mid eleventh century and the Waldensians in the twelfth century. And of course the preaching orders of Franciscans and Dominicans later in the thirteenth century. Plus many more, most of which we know very little about, sometime just a name, if that. There was a need for lay devotion that was not met by the church, and all these movements were expression of that devotional need. Until the Franciscans, the church had opposed all these movements, and the opposition got stronger as the church's power became more centralized and the church defined itself more and more in terms that would put all these movements outside the mainstream, as heretics.
We don't know much about the Cathar's theology, and the most complex accounts of their beliefs come from inquisition records. Was the theology we read about their own or was it something that the inquisitors suggested? It is likely that they did not recognize the church hierarchy and some of the rites, like infant baptism, but when we have inquisition accounts of rather simple folks admitting to fairly complex heretical theological arguments about the nature of God and Christ, those accounts become questionable.
There is a lot written about the Albigensian crusade and whether is was truly a religious crusade against heretics or just an excuse for some violent political realignment in the south of France. Either way, it is worth noting that there was no crusade against the Cathars in Italy, and that the Italian Cathars seem to disappear by the end of the thirteenth century, just as the preaching orders get approval from the church and become very popular elements of religious devotion all over Europe. It is possible that the Cathars were not really decimated by the Albigensian crusade, but rather by the introduction of church-approved forms of lay devotion.