system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Dark Apostles are said to be capable of making such persuasive and impassioned sermons whole regiments of Imperial Guards are made to embrace Chaos. What kind of speeches/arguments do they usually make?
gpt
It actually depends. The Imperial Guard number in the hundreds of billions and the Imperium is gigantic and widespread. But the argument can break down into a few things:

* The Imperium sucks, if it didn't why were you forced into being a soldier?

* The God Emperor is a powerless corpse, even if he did have powers why does your military and/or civilian life suck?

* The Chaos Gods are unholy, but you're eventually going to die so you might as well try and gain their favor in life.

* The Chaos Gods are holy, why do you think the might of the entire Imperium, Eldar, Necrons, and possibly Tyranids and Tau can't stop them? They're the good guys and good guys always win.

* They don't actually make a compelling argument of any kind, but given power by Chaos that corrupts anyone within earshot when they speak. (I think only Slaanesh and maybe Tzneentch followers have this power)

EDIT: In greater detail, everyone's life in the Imperium is really terrible. If it isn't, you're either blissfully unaware on an undeveloped world, lucky enough to be born on a developed peaceful world, or by the Grace of the Emperor and unfathomable luck born to a family with immense power and wealth so you're pampered from the day you're born to the day you die. Even then you're subject to invasion, Chaos corruption, an Inquisitor, or just down right bad luck.

There's also the fact the Chaos Gods aren't "evil" in the traditional sense, but positive traits taken to the greatest extreme.

* Slaanesh is when anything "happy" becomes obsession.

* Tzneentch is when knowledge and wisdom consumes the pursuer.

* Nurgle is friendship warped into decay and destruction.

* Khorne is courage, valor, and everything in between becomes bloodlust and war.