The main idea behind warhammer 40k was grimdark, a future when humanity lives in a sci-fi dark age where war is constant and most people live in 1800s industrial city living conditions. As such the Imperium was written to be "the most evil good guys"; they are overtly religiously zealous (in a way that doesn't even make sense because the Emperor they worship denied his divinity himself), they slaughter their own civilians en masse, they commit abhorrent war crimes and they genocide any alien race they encounter. Problem is at the end of the day they were the good guys, everything evil they did was either portrayed as a necessary evil or a result of humanity's very tumultuous history. Then they started to tone down the grimdark stuff a bit when 40k lore expanded beyond just being a background for a tabletop game and the suspension of disbelief increased. Then they went even further and actually played into the religious stuff, making things like imperial saints and even floating the possibility of the Emperor actually being a god after all. So after all the Imperium may have be awful, but it is awful in order to survive in a galaxy as horrible and violent as that of 40k. Obviously this creates a very chuddy message, borderline fascistic even (of course fascistic by the modern meaning of that word), which is pretty based and is one of the reasons why I like the franchise, even though a lot of stupid neckbeards will say that 40k is "woke" or "atheist" because the emperor hated religion or whatever. However for GW this obviously creates a massive problem in the modern day and age, so now on one hand they are trying to say that the imperium is "muh evil" and you shouldn't idolize it, but on the other they are introducing shit like female custodes or greater ethnic diversity amongst space marines. They really did corner themselves in this situation because they need to have the imperium as the protagonists but at the same time to keep the appeal of the franchise they have to keep it edgy and grimdark at least somewhat. They can't even write shit like the Emperor returning an waging war on the ecclesiarchy because there are imperial saints now.