Once the Latin Church figured out how to justify slaughtering people who weren't believing the things they were supposed to believe, according to the Latin Church, it was a short leap from slaughtering them in the Holy Land to slaughtering them in Europe.  The Cathars were being very wrong, very wrong indeed, on account of being dualists and not believing in things like baptism and the resurrection. So the Pope called a crusade against them.  And the French monarchy was glad to help, since the Languedoc -- where most of the Cathars were hanging out -- was rich and enticing territory to annex.  To France. Which is why, in the Languedoc today, they mostly speak French rather than Occitan. Even though "languedoc" is from "langue d'oc"-- "language of òc."  That's one way languages get endangered.