system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] The Imperium comes across a planet where everyone knows about Chaos, what happens?

Ok, so the Imperium finds an as of yet discovered old human colony, around Terra circa 2016 in technology and, daemons and the warp are common knowledge there but, everybody despises them with the same fervor that the Imperium does. Does that make it Exterminatus time or not?
gpt
Exterminatus is only used if the world as actively hosting an enemy force which cannot be repelled or removed from the planet by other means.

If the world could be convinced to accept Imperium rule, a task force would be sent to sterilize and place the native population in work camps. Adeptus Mechanicus local branches would sweep over and pick out any useful technology for cataloging, Inquisitors burn any books or other data storage that might induce heretical thoughts, and then eventually new Imperial citizens are shipped in to colonize the world and turn it over to Imperium prodution. They are told that the surviving former inhabitants, in their work camps, are criminals and not to be sheltered or listened to. Eventually the old population dies out and is forgotten. This is basically what the Inquisition tried to do on Armageddon, where the native (loyalist) population had become widely aware of Chaos.

If the world cannot be convinced to accept Imperial rule, they're obviously enemies of the Imperium and deserve no mercy. The world is invaded, the inhabitants slaughtered, and proceed as above.