The Scarlet Sign, as the are also known, have placed spies, assassins, and sleeper agents throughout the continent to manipulate and destabilize other countries with the goal of subjugating all other peoples to their land. However, they're not just D&D's equivalent of
Cobra with a dash of the Crusade-era
Ḥashīshiyyīn thrown in for flavor. What really makes them interesting is that the are racial supremacists (the Suel human ethnicity) that place no value on the lives of non-Suel humans, demi-humans, humanoids, or any other sapient species and barely any for non-pure-blooded Suel to such an extent that they not only keep non-Suel as slaves, they freely perform (typically magical) experients upon non-Suel slaves and prisoners and breed and cross-breed them (often with the aid of magic) to create servitor species. The Derro (featured in Mordenkainen's Tome of Foes) are, in Greyhawk at least, one of such servitor species that the Suel created (though that happened in the distant past). Oh, and there's also supposed to be a secret sect within the Scarlet brotherhood that worships Tharizdun (Greyhawk's cosmic horror, name-checked in the warlock class' Great Old One patron entry), so there's also that angle. It's very easy to explore that inherent darkness of the Scarlet Brotherhood and bring it out to the foreground.