Corax is one of the good guys. He buys into the vision of Emperor of Mankind, or at least Corax's interpretation of it.
'We shall use your facilties,' said Corax, for your cities will be part of the Imperium of Mankind.'
He also cares for the average human:
Corax glanced up, suddenly aware of the man's pain.
...or at least what Corax's interpretation of caring is. Corax has empathy of a sort but, being a Primarch, he expects that he knows best how humans feel, how they should react, and how they should be punished.
Haley is having some fun. Because in this reality, Corax is wrong. Alot. He's cleverish, winning the first three wargames against Guilliman (before losing the next seventeen), but latsr Corax's human subordinate doesn't hesitate to tell Corax how to fight an actual war.
Of course Corax will do things his way. Corax uses intimidation. It does not work. Corax uses stealth. It does not work. Corax uses shock warfare. It really really does not work. When he passes judgement on the recalcitrant:
"As the temperature rises within your refuge, and your flesh roasts upon your bones, you shall see why this had to be done."
...his enemies react by closing ranks and fighting grimly to the end.
Corax isn't a nimcompoop or an obvious incompetent like Konrad or Angron. He has aggressive sweeping plans and executes them. He reacts decisively to setbacks. The problem instead is that his interpretation of the Imperium's vision and human feelings isn't quite right.
He did not understand truth is subjective, not an absolute.
Hubris meeting nemesis is a common theme amongst the Primarchs, but the literary execution can be variable. This one keeps it simple and focused, tells you what's happening and why and, most importantly, suggests that the suffering of others due to Corax's actions doesn't actually register with him.
A man who wants to be good, but just doesn't quite get what being good is.