"them doing it freely or not is not the point" Yes, actually it is. Because if they are doing it freely, that's not slavery. Nevermind touching on the Undivided Princes or the fact that Primarchs like Fulgrim have apparently been doing nothing but enjoying themselves a personal pleasure planet for ten thousand years. The evidence that Daemon Princedom has actual rewards is pretty much everywhere in the setting. People would not follow Chaos if Chaos was not tempting, and Chaos cannot be tempting if there's no actual carrot on the end of the stick. Do a lot of Chaos followers die before they become Daemon Princes? Certainly. But are Daemon Princes slaves? There's no backing for that, and plenty of backing against it.
and what about all the cases where demons snd chaos gods lied and tricked people in become chaos servant? when demons mask their true nature or when chaos gods create plans for force mortals on becoming their follower? about eldar gods: eldars are an artificial race created by the old one, and my opinion is that eldars gods was created for the same reasons as a weapon for fight c'tan (or they even are survived old ones).
The only way to proove one servant is not a chaos god puppet is certainly not by serving them : their "free will" would look like way too much to mere illusion, since they could have been deceived from the start. This is the vast majority of cases actually : promise is freedom, power and/or immortality but aftermaths are quite more likely eternal slavery, madness, pain and/or mere annihilation... That's the very definition of deception.
Again you're telling me things without any proof. Most Daemon Princess don't appear to hate their existence, in cases of the Daemon Primarchs they also appear to largely retain their personality. If you can produce a contract showing that Daemon Princes are somehow bound to some eternal servitude i'd believe you, but i'v scoured the lore and i'm pretty sure such a contract does not exist. Furthermore what counts as 'Serving their will' to you? Is Fulgrim spending 10,000 years on a pleasure planet of Slaanesh apparently doing nothing the will of Slaanesh? what about the extremely personal goal of reuniting the World Eaters from Angron during the Dominion of Fire and First War of Armageddon? Perturbo seems largely apathetic to religion and has been managing the Iron Warriors from Medengarde for some time now, and countless other examples. The Gods do not descend on high to give them these tasks, because the Gods aren't really singular entities who can do that kind of thing, they are a trillion different thoughts gathered over the eons and even if they were just Rage, Excess, Scheming, and Despair their thoughts wouldn't be unified enough for such a one on one personal conversation....at least not in any capacity humans could possibly understand. Then they are slaves, you'll note that at no point do I deny that is a thing that happens. But it's not the *only* thing that happens, because that's dumb.
Chaos exists to feed on the eternal suffering of a universe locked in never ending war. It does not matter who serves who or when the first feral tribesmen brained another with a rock in blind rage, giving substance to the god of rage. The insubstantial beings of the warp are beyond mortal comprehension and those that strive for it invites insanity into their minds.
"I was born the sixth son of Kaschada Hetman of the Tabor on the day that the Great Prophet came among us. The Tabor were the outcasts of the world, condemned because we were faithful to the Gods of the Four Winds: The Blood Wind that fired the warrior's soul, the Plague Wind that purged the weak, the Wind of Change that brought the gifts of the Gods to men and the Scented Wind that roused the passions." from page 34 3rd edition Chaos Space Marine codex. The rest of the story is about a "cultist's" ascension to become a Black Legionaire (I've been thinking that this story was from a Word Bearer's perspective for a really long time due to the religious nature of the text, but it specifies Black Legion later in the story) It could be argued that their beliefs were misguided. But it could also be argued that because their emotions revolving around those gods were positive, it would influence those gods in a positive way due to the nature of their construction, albeit in an infinitely small way. The type of belief I mention above is rare from my readings of 40k books, but I prefer it over the boring stereotypical evil dude portrayal.
