I also like Sanguinius' reaction to his own power in a later scene (Because like, none of the Primarchs except maybe Magnus are aware of how powerful they truly are during the Heresy).
When the gravitational hold of the fortress weakened, and the fleets prepared to enter the warp again, Sanguinius called Mkani Kano to the Sanctorum Angelus. Sanguinius looked at the golden sculptures that made the space into an oasis of meditation, unable to draw peace from them.
'Do you know how we won on Pyrrhan?' Sanguinius asked.
'I do not,' Kano confessed.
Sanguinius told the Librarian a version of what he had experienced. He was circumspect about the visions. 'I saw visions of defeat,' he said. 'I found a single one of victory, and chose it.' He described the great fall, and the cleaving of the knots.
'Then you destroyed the manufactorum,' Kano said, awed.
'And by extension, the fortress gate itself. With a single blow. What I saw has given me great hope. But the implications trouble me, too. I must not believe I wield such cataclysmic power. No one should. The idea is corrupt and corrupting.'
Kano shook his head. 'The things we are seeing are beyond anyone's experience. The scale is simply too vast for easy explanation. They seem more symbolic than real.' He paused.
'Yes,' said Sanguinius. 'Symbolism. That is a key somehow. In the Signus System, that symbol of the eight-pointed star covered a planet. And there are patterns to the beings of the daemons. They represent things.'
'Then perhaps so did your victory,' Kano suggested. 'We can use their nature against them.'
As they do to us, Sanguinius thought. 'A single blow can change the galaxy, then,' he said. Or a single choice. That came before the blow.
'For the better.'
'In this case.' Sanguinius was not satisfied. Too much still flowed from his actions. He dreaded the constellation of circumstances that might grant him even more power.
'More reasons to be on guard,' said Kano. 'Even so…' He trailed off. Blood drained from his face. His eyes widened in sudden pain.
The Red Tear jerked, a toy in the hand of a giant. The drop from the warp was as violent as it had been before the fortress. The sound pierced the hull in the next moment. High-pitched, vibrating, like a tuning fork struck against madness itself.
Sanguinius supported Kano from the Sanctorum, fighting through the thrumming in his own skull. He felt warmth on his neck. Blood was running from his ears.
He refuses to believe that he wields such cataclysmic power because he doesn't want to look at himself as a god, and instead denies that he was responsible for the Fortress' destruction.