One question, why are you asking a 40k question in the WHFB section? Also, from the point of view of someone righting a [B]regular[B] guard story, think of how they could become tainted. A chaos sorcerer worked his way into the planetary government and began to turn the governors/officials to worshiping Tzeentch. Or (this one works for any god) the regiment was liberating a chaos held world, and while attacking a chaos temple became corrupted. Or while liberating the aforementioned planet glimpsed inside of a portal into the warp. The possibilities are endless, and any one of them could make sense.
Ooh, just got a good idea, after being attacked by cultists, the colonel/commisar was possessed by a daemon. Hope this helps.
Aerith/Aeris Gainsborough: “The current system of servitude is barbaric and inadequate for today's generation. We appeal to the leaders of both the Imperium and the Coalition of Hateful Assholes and Organised Sin to seek to change this for the betterment of all its citizens”
Lorgar: “Guess there was one plot thread we forgot to erase ...” Mortarion nods in agreement, while Ahriman looks on suspiciously.
Aerith/Aeris Gainsborough: “We feel trapped, we feel oppressed. And we urge our governments to release us from these metal boxes-”
Lorgar: “Oh shi-”
Firraveus Carron: Leaping to his feet. “METAL BOXES?!?METAL BOXES!!!THEY RIDE IN METAL BOXES!!!”
All credit goes to Lastie on Warseer, for writing Primarchs (available in a 647 page Microsoft Word document)