Seemingly devoted and purehearted members of the Imperium can unwittingly bolster the forces of Chaos and their followers. This isn't to say that every human that has sex channels power to Slaanesh, or that every battle the Imperial Guard engages in brings a Bloodthirster that much closer to manifestation.
But the noble whose ennui drives him to experiment with thoroughly heinous sexual practices, or the Lord-General who refuses to admit defeat and sends hundreds of thousands of Guardsmen to their doom on a daily basis: these are the people that can subvert the faith of the Imperium and bring power to the dark gods, without their realizing it.
This is purely my interpretation - no doubt others would argue that even seemingly innocuous acts can add to the collective power of the dark gods, and I think that is a perfectly valid view. Humanity struggling with its inner demons is a huge part of Warhammer 40K, and the forces of Chaos are both figurative and literal manifestations of these inner demons. Humanity slowly sliding into Chaos by simply being human is a delicious irony, and a theme that would no doubt be a hit at many game tables.