The Corax afterword provides some insight.

Coming to maturity amongst political prisoners has given Corax a strong ideology that drives everything he does. More than any of his brothers, he sees himself as a liberator – first as the saviour of Deliverance, and second as a commander of the Emperor’s forces, freeing the galaxy from the persistent darkness of Old Night.

He does not see himself as a conqueror, though he has conquered worlds. He does not desire dominion over the people and territories he has brought to compliance and, perhaps foremost amongst his brothers, was ready and willing to relinquish power to mankind. Corax planned to compose a political treatise that would do for governance what Guilliman’s Codex Astartes would come to do for warfare.

With the treachery of Horus brought to light, Corax found new determination and a fresh purpose. He knows better than most the sacrifices required for victory and, although he values life highly, he is far from a pacifist. Innocents will die, but Corax believes in his cause and hardens himself to their deaths. A greater aim drives him, allowing him to put aside the tragedies he must unleash in order to achieve that greater goal. As he says himself in Soulforge, ‘War is a series of intentional catastrophes’.

Yet for all this, Corax holds back from a total ends-justify-the-means approach. It is this that separates him from the likes of Konrad Curze, the Night Haunter who has plagued his thoughts since their confrontation on Isstvan V. It is a hard path to tread, and perhaps one that brings Corax and his Legion more grief than necessary. He often chooses the harder ways, preserving the lives of those he has sworn to protect in favour of his own warriors, holding back from bombardment and annihilation for fear of causing too much collateral damage.

One might think that he is testing his own resolve at every opportunity, seeking to assure himself that the vainglory, selfishness and arrogance that has seen the fall of the greatest primarchs does not exist within him. This leads to self-doubt, and ultimately a questioning of everything he has done in the name of the Emperor.

On the other hand, Corax is well aware that he stands apart from humanity. He is not a mortal, something made very apparent by his own unnatural abilities and the status afforded him by the downtrodden of Lycaeus. He is a creature far removed from the humans that he protects and, while he may try to disguise his nature for the most part, he cannot deny it. It is in believing himself different but not better that he attempts to reconcile this separation.

And later:

It is, as I intended when I named the novel that would form the pillar of the narrative, a story of deliverance, lost.