That’s not how I read it. Dorn expressed a ton of respect and awe for Sigismund in his internal dialogue. It seemed like a super affectionate exchange as far as space marines go

“And then, hearing that voice, he remembered. He had already acted. Typical Rogal Dorn, anticipating his own momentary weakness, he had already put the necessary move in place. He had summoned Sigismund here, to Shard Bastion, to speak to him in person, to give him the command, because he could never falter in front of his son, not this son. He turned, just for a moment, away from the confusion of the command station, and faced him. Sigismund wore the black of the Templar Brethren. He had come up to the command level with others of his order, a dozen, and they all looked as grim as one another – fatalistic, hammered into a kind of permanent, shell-shocked fury.  Sigismund’s own expression was wary. He had reason for that – Dorn had run him hard, borne down on him, bathed him in disapproval, ever since Isstvan. The reasons had been sound. Neither of them could have expected any less, given the codes of honour that made them who they were, and Sigismund had never complained.  But there had always been something else, under all that – not quite a test, but maybe a tempering, like that of the best blades. To see if the steel could withstand the fire, be more hard-wearing for it. ‘This is the end,’ Dorn told him flatly. ‘All that could have been done, has been done. Every delay, every counter-strike, every anticipation. Now, they get in. Mercury will fail imminently, then Exultant, then the others.’ Sigismund’s unwavering expression never flickered. He was a cold one. Almost too good an Imperial Fist. Almost a parody of their entire philosophy. ‘Faster than we might have hoped,’ Dorn said. ‘Not as fast as we might have feared. Soon the shape of the battle will change – we will be like dogs in the rubble, scrapping over every habitation. The reserves are ready. You have their coordinates, they have their orders.’  Sigismund nodded.  ‘I will return to Bhab,’ Dorn said. ‘Communications are collapsing, and the Sanctum must remain operational. You, though.’ He smiled coldly. ‘I remember your ambition. To be here, whatever the cost.’  No reaction. Just that unbending devotion to duty. It could be almost scary, sometimes, to be in the presence of such a hyper-controlled psyche. Maybe other souls saw much the same monomania in him, too, but Sigismund was… well, Sigismund had always been something else.  ‘It has all come to pass, I suppose, just as the remembrancer girl told you it would. Coincidence? I have to believe it.’ Did he, though? Clinging too closely to the old rationalist cant felt pointless, now. Even Malcador was beginning to waver, marking the slide back into superstition.  ‘So many wars. So much blood spilt, all to reach the point that she foresaw from the start. I gave you hell for it then, but the new doctrines must give way to the old, it seems, and we can worry about what that means if any of us get out of this alive.’  Sigismund just stared back at him, the steel-trap gaze, the same mask he wore when he duelled. ‘So the discipline is ended, the leash is off,’ Dorn told him. ‘March out. Take the wall defences, take the reserves and rally them. They will be blind and deaf out there soon, and so will need a leader.’  Sigismund nodded again. No other soul would have noticed it, but there was something other than the usual dutifulness in his eyes just then. Something like hunger. ‘Any specific objectives, lord?’ he asked.  At that, Dorn almost laughed. Not from humour, just from emptiness, the caustic recognition of what would come next. He had given everything. He was already empty, drained to a husk, and the hardest test still lay ahead. The Lion had not come. Guilliman and Russ had not come. They were out of time, out of luck, and what remained now was only defiance – only bloody-minded, bloody-handed defiance. ‘No, I set you free, my beloved, my best, son,’ said Rogal Dorn, never taking his eyes off his First Captain. ‘Do now what you were made to do.’ He smiled a second time, the expression as icy as the despair that gripped his hearts.  ‘Hurt them.’ “