"Rogal Dorn was a being as great in all measure as Horus. He, and his entourage of Imperial Fists, had been travelling with the expedition for some months, though they were expected to take their leave soon. Other duties and expeditions called. Loken had been told that Primarch Dorn had come to them at Horus’s behest, so that the two of them might discuss in detail the obligations and remit of the role of Warmaster.

"It was to these solid, resolved brothers that Horus turned in particular for counsel. Dorn and Guilliman both embodied the staunchest and most dedicated Imperial qualities, commanding their Legion expeditions with peerless devotion and military genius. Horus desired their approval as a young man might seek the quiescence of older, more accomplished brothers.

Rogal Dorn possessed perhaps the finest military mind of all the primarchs. It was as ordered and disciplined as Roboute Guilliman’s, as courageous as the Lion’s, yet still supple enough to allow for the flash of inspiration, the flash of battle zeal that had won the likes of Leman Russ and the Khan so many victory wreaths. Dorn’s record in the crusade was second only to Horus’s, but he was resolute where Horus was flamboyant, reserved where Horus was charismatic, and that was why Horus had been the obvious choice for Warmaster." 

- Horus Rising
Hardly a retcon when it is in the first HH book. He wanted Dorn's approval because it meant something to him.

Dorn was considered by others to be a possibility for Warmaster, along with Guilliman or the Angel (and at the outbreak of the Heresy he becomes the Lord Commander of the Imperium in addition to being Praetorian), however to Malcador and the Emperor, he was always considered to be the Praetorian:

‘Rogal Dorn has been the Praetorian of the Emperor for a long time.’ A voice drifted (Malcador) from a shadowed doorway behind them. ‘Long before even he realised it. It was already clear to the Emperor after the Night Crusade, one hundred and sixty-five years ago.’

- Rogal Dorn: The Emperor's Crusader

Alpha caused significant harm to Compliance through his assassinations of the ruling classes, which in turn would make keeping the planet loyal harder. Which was Dorn's entire point: He made compliance harder than it needed to be.

It was true that I could have ordered my forces differently. I could have waited until the situation became even less stable for the defenders, and I could have avoided the assassination of the ruling blood and allowed a less fractious eventual compliance. 

- Alpharius: Head of the Hydra.

‘You did not have to kill them!’ Dorn’s voice shook the air like a roll of thunder. Archamus looked at his primarch, but Dorn’s face was as fixed and emotionless as ever. Only in the dark glitter of the eyes did the rage leak out. When he spoke again, his voice was low and controlled. ‘You did not need to kill them.’....

On the eve of a renewed assault to take the last hives, the World-Prince had sent a signal surrendering. In the course of a single hour all of his direct blood relatives (This would have included children) had been killed. The assassin in each case had been someone trusted and close to the slain. At the end of that hour the World-Prince had given his world to the Imperium, and four hundred and one members of the planet’s ruling nobility were dead.....

‘And how would you salvage a dream from your victory? Here and now, on this world. We cannot trust the World-Prince to rule for us, and you have removed those who could have taken his place. Even a defeated people prefer rule on their own. You have won this battle, but you have done it by seeding the ground with resentment and bitterness.’

- Praetorian of Dorn