So let's say that there are two large populations of an endangered animal left in the world. The largest is in Sanctuary A, and a smaller but still viable population lives in Sanctuary B. You are in charge of Sanctuary B, and have sworn oaths to protect this endangered animal at all costs. Suddenly, you lose contact with not only Sanctuary A, but most of the known world. You can't verify if anything actually happened out there, or whether Sanctuary A is still out there. They could be perfectly fine, or you could be the Steward of the only remaining population of the species you are sworn to protect. You've tried calling, but nobody picks up the phone. You've tried sending interns to check on them, but you never hear from them again either. You even dig up an old shortwave radio but still can't get any information on Sanctuary A or the wider world outside. So, being sworn to protect this species, you have two options:
Option 1, you assume the worst has happened. If it did, you are now nurturing the only large population of this animal. If it did not, then both populations are still alive and well.
Option 2, you assume the best has happened. If it did, then you sacrifice your population to go and aid the larger population. If it didn't, you sacrifice your population, and the animal is now extinct.
The fanatics among us, who give more loyalty to the Sanctuary Preservation Corporation than to the endangered species itself may say that you go check on Sanctuary A and the head offices, and make sure that the company is doing well. Those who are legitimately loyal to the endangered species, however, will do what is necessary to preserve the species, even if it may not be in the best interests of the company running the Sanctuary. Guilliman was and is loyal to humanity first, and The Emperor second. Given the choice between loyalties, he chose the option with the greatest potential for humanity to survive.
Two common threads among many Primarchs are either a warrior culture, or fanatacism. These are the two main things that led to most falls to chaos. Magnus was fanatical about knowledge, Fulgrim about perfection, Lorgar about worship. Angron was obsessed with warrior prowess, as were many other Primarchs, even loyalists. Guilliman didn't really fit in. He was a statesman. He was the boring kid who becomes high school class president because everyone admits that, yeah, he'd probably be really good at it and nobody else really wants to. It seems to me that he wasn't necessarily beloved by all his brothers, but only a few seemed to actively dislike him. Being able to fly? That's awesome. Unparalleled sorcerer? Awesome. Clouding minds to hide your presence? Awwwweeessssoooommmmeeeee. Being really good at juggling thousands of supply lines, diplomatic envoys, and trade agreements? LAAAAMMMMMMEEEEEE.
Guilliman's sin isn't being derelict in duty, or deficient in prowess as a military Commander. It's that he was boring.
Fortnight, Walter Payton, Loquille and 2 others like this