As a matter of historical interest "Kill Them All and Let God Sort Them Out" was a much used phrase in "nam, I guess.
It was first used, in French or latin, I assume, at the siege of a town in "Languedoc" during the Albigensian Crusade. This was a particularly bad offshot of the Crusading impulse (I am Catholic-but this was bad). The Cathars were a medium sized sect on the 13th Century in southern France who major "mistake" seemed to be in interpreting scripture to find "no authority" for a Papacy. This was often a fatal error in medieval Europe.
As it turned out, the Pope called a Crusade against fellow Chrsitians, whose main center was the town of "Albi" hence the name of the Crusade. Thousand of mostly Northern French, maybe Belgians and Germans thronged for the Fight, mostly, it must be said, for loot and a chance to "settle scores" with Languedoc.
The Papal Legate managed to (with no military experience I am told, but I could be wrong) appointed commander of one of the
"Crusading" Armies. The y laid siege to Beziers, a city of perhaps 30,000. In the town there were perhaps 300 Cathars. To their credit, and Like the Danes in WWII with respect to their Jewish Population, the Town refused to turn over the Cathars.
Thus the "Bishop" was repute to have said Kill Them All and Let Them Stand Before God", and it was done.