Horus Heresy (do yourself a favor and skip 'battle for the abyss', and unless you are willing to take a hit to common sense and lovely old fluff 'descent of angels')
Eisenhorne (xenos/malleus/hereticus) series
Those give you a real feel for the setup of WHY in the 40k universe.
the fall, and then the paranoia.
"You've heard of animals chewing off a leg to escape a trap? There's an animal kind of trick. A human would remain in the trap, endure the pain, feigning death that he might kill the trapper and remove a threat to his kind."
"The best years are the forties; after fifty a man begins to deteriorate, but in the forties he is at the maximum of his villainy."