I think if they weren't always this vile, it was pretty quick. like maybe their ancestors had somewhat of a point during the war, but had one point towards the end that cemented them as evil, like an extermination campaign of nonhuman children
Also if the national treasure of mariegeoise is related to the grimoire of imu that can turn men into devils, they may have made a deal like we saw the people of that island brook went to were willing to make.