Read the Word Bearers trilogy by Anthony Reynolds - that exact scenario effectively happens (the Dark Apostle is murdered by his First Acolyte after trying to murder him, but the host doesn't know the apostle was murdered). Dark Apostles are appointed by the Dark Council on Sicarius, and it would take a lot for the members of a host to rebel against their Dark Apostle.
There are a lot of factors that would go into answering it
- If host is rebelling against their Dark Apostle, why?
- Who murders the apostle, and regardless of who it was, is their favour with the gods high? Were they seen as his successor? Could it be seen as fate/prophecy/the will of the gods?
Perhaps if an Apostle has led them into disaster after disaster and has obviously lost the favour of the gods, his acolyte might lead a rebellion against him and before leading the host back to their homeworld to seek confirmation from the Dark Council. This is all just based off Reynolds' series
I'm just formulating a series of what-if scenarios in my head. I kinda base anything I do on "what ifs". My Tau army that I just started, in my head cannon, are rebels against the Tau Empire looking to eliminate the Caste System and murder the Ethereals for being "enemies of the people", for instance.
But, as a long time Word Bearers player and fan, one thing I find interesting (which I love about them too) is the fact the Word Bearers are kinda :cuss-heels, and of all the Legions / Warbands out there, you should never trust the Word, of a Word Bearer. The Dark Apostles sell their own down the river pretty routinely. They sell their allies out just as easily. Everything is about manipulation and control, and I find a lot of their leaders self-interested, as much as they are true believers. Which I find to be truly endearing, I actually really dig it.
I write just for exercises for my brain, honestly, for the most part. So I kinda got to thinking about making a character, whose not an Apostle or set to be one, who isn't a Veteran of the Long War, who is a fanatical worshiper of Chaos, but is a character who is... virtuous (for lack of a better term). The bad guys never think of themselves as bad-guys. They often see themselves in the lens of the Hero. So I'm intending to write a character who kinda embodies something which is very foreign to Chaos, or it has been since the Heresy. Which is someone who fanatically believes in the Dark Gods and the teachings of Lorgar, but someone who believes in being honest, and having a degree of integrity. Which I genuinely just don't see a lot of in the Word Bearers (which is awesome, might I add).
Which also ends up being the Catalyst for the story, of course. Given such a character could never operate in such a Warband, at least not for long. Which of course leads to the confrontation which leads to the elimination of his Apostle. It's very much aiming to be a story about liberation and self-determination, as well as the question of loyalty and power. Now, this sounds like he's a great individual (he's not), the story is going to depict him as the kind of monster a Chaos Space Marine is meant to be, but at the same time there are humanizing traits in the primary character.
It's just something I'm toying with. But spoilers, a Dark Apostle may get killed.