Board Thread:Warhammer 40k General Discussion/@comment-24821739-20140604204305/@comment-24944355-20140618012852

The Inquisition would never accept any attempt to reform it, even by a primarch. If Guilliman - the most respected of the primarchs after Sanguinius - was healed and tried to repair the damage to the imperium by reforming the Inquisition, along with the other departments, some of the individual inquisitors would accept his judgements but the Inquisition as a whole would not. They would declare Guilliman a chaos spawned fake and try to destroy him, even to the point of civil war.

Basically the Inquisition has been infected by the same hubris that was a large part of the reason that the traitor legions turned on the Emperor. Which is that they were responsible for creating the Empire and they should rule it and no other. It was the administrators from Terra coming in and telling them what to do that was a driving force behind the turning of several legions, and the Inquisition has the same attitude.