but with Exterminatus looming over the heads of the poor people of the Imperium,
Except that Exterminatus isn't a threat that hangs over the heads of the people of the Imperium. The average citizen has no knowledge of Exterminatus, and wouldn't understand the word if it was used as a threat. And it would be a poor Inquisitor who blustered about the Imperium's ultimate sanction. Even in the vast Imperium of Man, Exterminatus is not to be used lightly. Unless GW have significantly changed the fluff recently, only an Inquisitor can order Exterminatus, so it isn't an option for Space Marines (at any level of OOB).
Chaos is inherent in humanity - since Chaos represents (or represented) change and destruction. Which is why so many humans fall, whereas Orks are a force of chaos, and whilst their actions are certainly Chaotic on occasion, the Orky psyche is more self-sufficient than the Human and you don't see as many Ork cultists - except for the Stormboyz of Khorne, and Weirdboy Warpheadz. Ork society tends to ostracise Chaos worshippers as un-Orky.
It's debatable whether Kaela Mensha Khaine is Khorne. I say not, Khaine is only an aspect of Khorne in the Warhammer World, as Lord of Murder. I don't think the Eldar worship Khorne in any sense. Although I'd have to trawl through the Eldar fluff some to work out their relationship with Slaanesh again, beyond, they created it somehow and they are mortal enemies, and Slaanesh is the whole reason for soul-stones.
Dark Eldar are definitely Chaotic.
Space Marines are obviously vulnerable to the taint of Chaos - cf the Horus Heresy and the Fallen. Many might also argue that Sanguinus and the Blood Angels are tainted by Chaos. Sanguinus was a mutant, and the Red Thirst is possibly a mutation.
Necrons. Pfah.
Ego Ordinis Mallei Inquisitor, per auctoritate Digamma, Decimatio, Duodecies, ultimum exterminatum planetae [insert here] cum extrema celeritate impero.
(roughly; 'I, Ordo Malleus Inquisitor, hereby authorise Death, Destruction, Doom, the total extermination of the planet "blah" with the most extreme and imperative speed.')
In a galaxy of a million worlds, what does the death of one matter?
Simguinus, Chief Historitor
http://rpg.drivethrustuff.com/catalog/p ... e_id=66353