Whenever natural disasters or man-made calamities struck the Muggle world and caused social unrest, some Dark wizards always took the opportunity to come out and make trouble.
In the 1340s, Nicholas Malfoy opened an inn and used it as a cover to kill many Muggle lodgers, blaming their deaths on the Black Death.
This was much like a serial killer. H. H. Holmes, America's "first" serial killer, predated Jack the Ripper by twenty years, and his method had been to build a hotel.
Whether Muggle or wizard serial killers, their targets were outsiders. They had no relatives in the area, and with the communications of the time, it often took about a year before anyone noticed they had vanished. By the time the missing person's family came looking, the evidence had long since been destroyed, and the killers could claim the lodger had gone elsewhere.
Even in the twenty-first century, missing-person cases were difficult to solve. The basis for distinguishing murder from disappearance was the corpse, which was why Voldemort allowed Werewolves to "serve" him.
When the early Scourers were still in Europe, they distinguished themselves from wizards—or rather, disguised themselves as "one of us." They helped Muggles eliminate the wizards who caused natural disasters and plagues, becoming "guarantors of the harvest." Some Werewolves seized the opportunity to claim they were God's hounds.
Sometimes people did not fail to distinguish friend from foe; there were simply too many wolves disguised as sheep, making them impossible to tell apart. In truth, whether Edward III had prohibited British wool exports to the Netherlands during the Hundred Years' War no longer mattered much, because the Netherlands was then suffering the Black Death, natural disasters, and a war of succession. The last male of the Avesnes family, which had originally ruled Flanders, invaded Friesland