It is dubiously alleged that prior to the massacre of Béziers, Amalric, when asked how to distinguish Cathars from Catholics, responded, "Kill them [all], for God knows which are His own."

In 1204, he was named a papal legate and inquisitor and was sent by Pope Innocent III with Peter of Castelnau and Arnoul to attempt the conversion of the Albigensians.

[2] There, according to the Cistercian writer Caesarius of Heisterbach, Arnaud responded when asked by a Crusader how to distinguish the Cathars from the Catholics: Caedite eos.

It is noticeable that most commentators insert “all” for the sake of emphasis and omit fertur dixisse by which Caesarius makes clear that this is hearsay.”[6] Laurence Marvin comments "in 1210 this inflexible and unyielding man gave Cathars who surrendered a fair chance to abjure their heresy and so avoid execution, which heaps more doubt on the credibility of Caesarius’ report.

Conversely, British Cathar historian James McDonald suggests that Caesarius may have only proffered such a caveat in an effort to obtain "plausible deniability."

Caesarius, an accomplished writer, knew that his Cistercian audience would be impressed by the words, but related the story in such a way as to leave plausible deniability.

Only Arnaud’s alleged answer is qualified – Caesarius reports the preceding question about how to distinguish Cathar from Catholic as a fact.

[8]Arnaud himself, in a letter to the Pope in August 1209 (col.139), wrote: ...dum tractatetur cum baronibus de liberatione illorum qui in civitate ipsa catholici censebantur, ribaldi et alii viles et inermes personæ, non exspectato mandato principum, in civitatem fecerunt insultum, et mirantibus nostris, cum clamaretur : Ad arma, ad arma, quasi sub duarum vel trium horarum spatio, transcensis fossatis ac muro, capta est civitas Biterrensis, nostrique non parcentes ordini, sexui, vel ætati, fere viginti millia hominum in ore gladii peremerunt; factaque hostium strage permaxima, spoliata est tota civitas et succensa...

After this great slaughter the whole city was despoiled and burnt...[9] Amalric's account of the death of 20,000 was likely exaggerated, just like Peter of Vaux de Cernay's report that 7,000 were slain in the Church of St Magdalene.

"[11] Historian Laurence W. Marvin calls Amalric's exhortation "apocryphal", adding that the "speed and spontaneity of the attack indicates that the legate may not have actually known what was going on until it was over".

"[12] After helping the Crusaders capture Carcassonne, Amalric was replaced as commander of the army by Simon de Montfort, 5th Earl of Leicester.