I have spent more time thinking about how Guilliman interacts with Fabian in Avenging Son (obviously cannot speak for anything in later books) and I really cannot stop thinking about the fact that he gifts Messinius weapons and offers him a choice in one scene and at the very end of the book he gifts Fabian weapons and Fabian is explicitly denied a choice throughout every scene he is present in the book
it really just says a lot about Guilliman's paternalism and benign control of people. The fact that he is capable of recognizing someone's autonomy, even more so in the context of Astartes who are explicitly stripped of it by design, to me means that the contrast with Fabian is intentional.
Obviously this to a degree says things about the role of baseline humans in the setting in general but I find it interesting how much it is reflected in the guy people are most likely to point at and call him good, because he really is authoritarian to the bone and this sort of Father As The State paternalism can easily be obscured by the character also being so damn nice about it.
He is kind, he is concerned, he is accommodating, all of that is real and shows genuine care on his part--but he has decided what your life is going to look like from here on out. Do not resist.
It's not that he doesn't have choice either in my eyes, at least not in the case of Fabian. He says himself that there are probably many with Fabian's talents, but Fabian happened to be there. He chose to essentially blackmail Fabian by presenting him with the illusion of a choice: come work for me or face the charges laid out against you. This most likely means death.
Guilliman is the Imperial Regent. He could have, most likely, gotten Fabian safely out of there if he wanted to. Using this as leverage was a choice. I frankly doubt it would even have cost him much extra.
He may hate what the Imperium has become but the root of it all is still in his bones and he will actively use the corruption if it serves his purpose, because Fabian's crime was simply to speak up over things that were true.
Obviously this is just how the Imperium is. But it is also in him, and I'm pretty sure it was already in him before the Heresy because he already had Father Knows Best moments then.
These scenes are so fun to read and I actually adore Guilliman in them because he is so civil and frankly funny but they also reveal so much about his character in such a short time it's almost painful.
(also something that stood out to me is how crazy Fabian really is because he looks and acts like a nervous wreck but he accidentally gives a walking demigod shit WHILE being actively psychically fried by said demigod. What an icon. )
