I would like a story of warhammer 40K where a cult begins, like a normal guy or girl down on his/her luck starting from being a righteous person but being forced to make wrong decisions for his/her and maybe some other's survival, slowly getting crueler and crueler untill he/she actually becomes one of the most powerful agents of chaos (or maybe not even a powerful one who knows maybe just a random cultist could make for a good story also).

In short, in the world where everything is black and white, some shades of gray.