Maybe give this a spin.
A minor tzeentchian cult springs up amoungst the nobles and merchants of the world,tired to being ignored and sidelined by the headstrong governor and by the sector in general due to the planet`s relative lack of wealth and isolation.
The cultists are trading artifacts with a nameless offworld source trying to gain that ever elusive quick route to greater power and influence, their obsession with their search for forbidden lore and artifacts causes them to neglect their people and duties causing the cult of the red redemption to gain a growing foothold among the dissastisfied populace that is slowly building into a popular uprising.
The governor, seeing the uprising on the horizon is urged by his tzeentchian advisors to suppress the unrest. He unleashes the PDF and secretly calls for assistance from the Imperium (enter the acolytes) little does he know that his violent campaign against the redemptionists will allow the secret khornate cult in their upper ranks to effectively declare war on the tzeentchian nobles.
The planet starts to tear itself apart as the khornates, backed up by the people hold an overwhelming advantage in numbers while the tzeenchians and the nobility use the full weight of the PDF, security forces, the near unlimited resources of the Governor`s household and their own dark lore to fight back against the tide.
The world falls ino swirling civil war that ultimately grinds to a stalemate, lawlessness and chaos reign.
Which is just what the Word Bearers hiding on a stolen ship in orbit had wanted all along. Seeing the world rendered defenseless by the cults they seeded decades before they launch their attack.
More shades of moral greyness, nobody is pure but the governor and the acolyes. Ultimately nobody is the good guy players must side with one faction or the other or risk being targeted by both.