The thing people commented on GW failing to show how evil the Imperium is supposed to be, it is also the same with how "inhuman" Marines and Custodes are supposed to be.
You got a Custodian in Wolftime talking on how "You got lucky, you would lose everything that makes you human," to a nobleman who was not given to the custodes, yet every single custode pov we see is extremely human. Even the idea of "They are Yes Men who can't say no to the Emperor" is moot because they can still disagree; they just physically can't go against his wishes.
Hell, people assume that the Sisters are "way more cruel than the average" because at least they are allowed to be written as monsters from time to time, yet the brainwashed child soldier whose body is as far from a normal human as an ork is remains just a dude who 9 times out of 10 is a classical hero.