If he could have, Guilliman would not have wanted to deal with the Black Templars.
From the bottom of his heart, he did not want any contact with a group of religious lunatics fanatical to the extreme, but the problem was that reality did not allow him to pick and choose.
Reality was just that cruel.
No one knew how angry Guilliman had been when he learned that the Black Templars had once wiped out every civilian on a planet because Psionic individuals had appeared there, and no one knew why he had made this decision.
The Ultramarines were rather puzzled by it, and Yvraine was also worried because of the Black Templars' reputation. She was afraid her kin would do something irrational after being provoked by them, and so she gave them a very careful set of instructions.
And she did it right in front of Sicarius.
But the Second Company Captain showed no dissatisfaction, for two reasons. First, he had long since become used to treating Yvraine's presence as air. Second, he was actually worried about this matter too.
Sicarius knew very well just how stubborn the Black Templars' faith was.
In truth, the Ultramarines did not have a particularly good impression of them. The reason was that more than once, they had killed every civilian on an entire planet over the slightest sign of Psionic individuals.
Moreover, Sicarius did not think his Primarch could possibly be unaware of this matter—since the Primarch knew, then could this order he had issued be a test?
Politics was filthy, but politics was also necessary. Sicarius understood that if the Black Templars remained obstinate, still stubbornly trying to place their faith before their orders, then some things would likely happen without the slightest resistance.
Back to the matter at hand. This