Radical

Within the Imperial Inquisition, Radicals are those agents of the Golden Throne who turn to methods, tools, and ideologies forbidden by the Imperial Creed, choosing instead to turn the darkness against itself.

This proves to be a slippery slope indeed, and thus Radicals tread where many others fear to follow, imperiling their very souls in the quest to find salvation for a million worlds.

Source

 * Dark Heresy (1st Edition): The Radical's Handbook (RPG), pg. 6