I have this crack theory about the relationship between Chaos and humanity and the Imperium in the Great Crusade era (not crack as in funny, as in angry and depressing but probably not canon).
So, Chaos hates the Emperor, the Anathema, and considers him and his works obstructions to its ability to do whatever it wants in the Materium and Immaterium. Okay, but I don’t think this fully explains the sheer virulence of the hatred Chaos bears for the Imperium, particularly the later Imperium which is a pretty shitty place Chaos should like on some level. Building a Webway, that means fewer tasty spaceships, but humans currently engaged in Warp travel were always a miniscule fraction of the galactic population and it’s not like it mattered with the Eldar. That’s not something that Chaos has reason to get nervous about.
I don’t necessarily see the Age of Strife as Chaos being malevolent towards humanity with any purpose or on any scale. That is, the warpstorms were more a thing that happened than a thing that happened for the purpose of screwing with humanity. The birth pains of Slaanesh were about the Eldar, and the fact that humanity’s ability to FTL travel was messed with too was coincidental. There were robot wars, but that’s just a thing that happens and if anything there’s very little sign of them being proto-Dark Mechanicum Chaos robots as opposed to normal killer robot robots. Psykers started appearing and opening daemon portals on their unsuspecting planets, but honestly that’s just a thing that happens when you have rogue psykers existing. Chaos had a presence among humanity, good old Lovecraftian religion as old as the human race, but if nothing else humans weren’t that psychic yet and I don’t think it cared that much. What changed?
I see a duality in Chaos: on one hand, it is sentient and has its own goals and purposes and so on. On the other hand, it is the collective unconscious of all life in the galaxy. The former and people being unaware of the former plays a big part in the Heresy books, but I want to talk about that latter aspect. That Chaos so deeply and personally hates the Imperium, that implies humanity hating itself. As below so above.
Which brings me to the Great Crusade. We don’t know much about the Dark Age of Technology, but I gather most of the fighting was done with robots anyway and I’m just going to go out on a limb here and call this the greatest slaughter of humans by humans that ever existed on a galactic scale, certainly since humans became reasonably psychic. There have been wars before, are wars all the time, but not on this scale, not with such a singular concentration of their hatred. My thesis here is that not just was the Imperium flawed for all the obvious reasons of being a bunch of intolerant, warmongering fanatics (just not religious fanatics yet), but just by conquering, they had already inherently lost.
Consider compliance. Consider the sheer loss of life. Consider the constant massacre, every time a world didn’t instantly fall over itself to join the Imperium or someone was in a hurry or they once talked to a xeno without shooting it and we can always recolonise it later with our own people. This killing of worlds, of course it had to have had an effect on the Immaterium.
Every scream of the soul, in Chaos are they heard. No, fuck this, I wanted to live. Fight or hide and die either way. I’d have joined them if they’d asked rather than this, but no one asked me. What went wrong, why is this happening? We got lucky–we’re citizens now, not enemies, but it’s still work in the mines and manufactorums all day and it’s treason to want anything else because the Crusade needs those bullets. I thought I was the hero of my own story and I tried so hard and overcome so much, but now I’m going to die as a footnote about a statistic in someone else’s. Those monsters, I want to kill them more than anything, but I can’t, I don’t have the power. What can the weak do to overcome the strong? I wanted my wife to live. I wouldn’t have minded dying half so much if she could live; I’d give anything for the power to protect her. They’re like locusts, they took everything–what will we eat now? Why are you doing this, this is stupid, whatever reasons or justifications you have are invalid because my daughter is dead. Help me. I lived, I mattered, I wanted to live longer, and I wanted those I love to live and to be free and to be happy. Fuck everything. Someone, something, anything, hear me. Over and over and over, from one end of the galaxy to the other. Crying out. Justice. Vengeance. Hatred.
Humanity creates its own gods.
because by fate even the gods are cast down 