Scourers were a group of mercenaries for the wizarding world acting as a form of goverment.
At the beginning, when it was first made the Scourers were a forced created to combat against crimes being committed by the new Settlers from Europe. Their job was to track down those with wanted posters as well as any other job needed. As time went on and their numbers grew and no real government was in play, they became killers and torturers. Scourers even went as far as to traffic their own wizards for money and even muggles.
The new Scourers were angry that there ancestors had been disbanded and even caught over the increased killing of muggles and the Salem witch Trials that they are dedicated to ruling over all muggles, while they do not hate half-bloods they also don't see the point in having a wizard be brought up by a muggle, even going as far as to kill the parents and take the child. One thing they are strongly interested in are the schools that the children are being taught at and have infiltrated some. They use the schools as a recruitment area trying to get kids that are alone and have nobody or are mistreated.
Ranking of Power
the leader of the Scourers has absolute power and what he wants done is done, without question. Next are his generals they hold a percentage of his army and have control of it. After the generals are the members they are people that have been picked up by the Scourers and have yet to earn any kind of title.
A special section that report directly to the leader are recruiters. These people have the job of enticing people to join, they typically will do it at a school as a teacher or member of staff.
During the 17th century, when North America was just starting to be settled by both non-magic and magic emigrants from Europe, the continent's wizarding community was small and fragmented. The absence of a centralized wizarding government meant that the North American wizarding community had no effective law enforcement mechanism. To fill this void, wizards and witches of many different nationalities banded together to form the Scourers, a mercenary group which acted as both vigilantes and bounty hunters, tracking down not only wanted criminals, but any person for whom a sufficient reward was offered.
As time went on and their numbers grew and no real government was in play, they became killers and torturers. Scourers even went as far as to traffic their own wizards for money and even muggles.
The newly formed MACUSA (Magical Congress of the United States of America) had enough of what was happening and started to track all Scourers and catch them. This resulted in them having to go into hiding by mixing with muggles. They would tell their descendants the story and how they were treated as if they were the good guys and the MACUSA as the bad guys.
Many Present day high ranking Scourers are direct descendants of the hunted Scourers, who fully believe in the story that their families have told.