The latter half of your comment is exactly the basis for the All Guardsman Party. It makes a ton of sense - you need people to do dirty deeds, you also need to disappear a bunch of guardsman, problem solved. Same as using convicts in Darktide.
Problem is that it makes too much sense for 40k. It’s gotta stay grimdark, doncha know? Also it’s directly in contradiction to lore from e.g. the conflict between the Space Wolves and the Inquisition.
I’ve always thought they should just start splitting the setting into more and less crazy versions. Or (gasp) do human factions that aren’t the Imperium. But that’s just my nonsense.
Yah, replace the UPS with USPS and it’d be bangin