The uniforms of Chaplains are not so much about terror as they are about being a walking visage of martyred saints and of the Emperor as a god of battle and death. The Emperor really wasn't big on terrorism-- that's why the Night Lords got in so much trouble and ended up renegade, and it's hard to see Guilliman going "Yeah, you know what we need? A bunch of psychopath stalkers with bigass shivs in stripped down power armor to go knife people in the dark." But that's exactly what the Reivers are-- and while I find them distinctly un-Imperial, maybe that's the point. Guilliman did refer to the Primaris marines as "Cawl's monstrosities" in the Dark Imperium novel, after all... there's that ambiguity about whether or not the Primaris Marines are even a good thing in the first place.
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