If we were writing a "Themes" section for the Dark Heresy GM book (like the sections in White Wolf books), what would we include? Here are some kickoff ideas - they may or may not be valid
Dark Heresy Themes for GM Section
Humanity Denied - Making personal connections in the fiction and burning them or watching them burn. Faking your death and denying yourself your family in the process. Slicing a suspect into tiny little pieces because in the grip of the Empyrean, no other means of interrogation exists. At what point do you deny your own humanity to the extent you become the enemy?
Body Horror - Sprouting new extremities that shouldn't be, carving off your limbs to replace them with metal, beating yourself to within an inch of your life in penance to the rotting corpse on Terra for some imagined failing. What boiling hell do you create of yourself working for the Inquisition?
Cosmic Horror - What moves beneath the Empyrean waves cannot be stopped. You cannot destroy the Daemon, merely delay it. Do you doom one world to save another? Or do you realise the other won't be saved, it's execution just postponed, and let unnature run its course?
Paranoia - When you're duty bound to kill to preserve the secrecy of all you do, but your superiors cannot be trusted and they put you together with your cell, how can you operate without killing the cell, your superiors, or getting killed by them? And what happens when external rivals turn up, who may or may not be on any kind of facet of your side?