system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Say a primarch either recovers from his wounds or otherwise returns to the Imperium. What impact would it have? What would the primarch's reaction be to what the Imperium has become?
gpt
It's dependant upon the Primarch. Those who always valued the life of mortals, such as Leman Russ, or Vulkan, would be disgusted by the disregard of human life and the rampant waste of it that is common nature to the Imperium. It is also safe to say that the religious fervor and devotion given to the Emperor and his Primarchs would create trouble. 

The Emperor was trying to spread the Imperial Truth: Mankind is the master of the galaxy by sheer power of will and virtue of strength and genetic perfection. There is no divine spark in the galaxy, no gods or demons, only small minds that are scared of the dark and the truth, seeking solace in these lies.

The fact that that man is now revered as a god and the cult is the state-enforced religion would be mind-boggling to a Primarch. Many, especially Guillaume, would also decry the mismanagement, the rampant Bureaucracy and corruption that has become the norm. 

Apart from that, the state of decay the Imperium is in would probably become their main focus and they would attempt to revive it in any way they seem reasonable, most likely starting with the Emperor himself. Corax may even go to far, he has already turned to dark means to recover from losses. He could potentially create another civil war by turning to dark powers to find a means to revive the Emperor.