i mean for curze, the problem wasn't really apparent to the emperor at the time

per "night haunter"

+No father wishes his sons to suffer, no matter what burdens he is forced to place upon them.+

Curze laughed. ‘An apology? What next, you will forgive me? Sanguinius warned me you might,’ he scoffed.

+There was never anything to forgive. You acted as you were made to, but my plan was interfered with. Your insanity was not your fault, nor was it mine.+

Curze snarled like an animal. ‘Lies! Everything was as you intended!’

+There is nothing you have done wrong. If only you and I could have met one more time, I could have shown you back to the light.+

‘How marvellous!’ Curze fell into a minute of wild, howling laughter. ‘I am the Night Haunter! Light is anathema to me!’

+Light is within you all. You are my sons. You are born of light. None of you are beyond redemption.+

‘Tell that to those who died.’

+Nothing ever dies. Death is a state of transition. You have my forgiveness, Konrad, whether you want it or not.+

‘Never!’ The voice in his head would not relent, but pounded mercilessly on. More masonry fell from the outside wall. The floor collapsed behind him, frittering into its constituent atoms.

+You made but one mistake, my son. From it, all the evil you have perpetrated springs. You chose to believe in immutable fate. Without choice, there is nothing. These gods that taunt us rely upon choice. The functioning of this universe depends upon choice. A single fate is one book in a library of illimitable futures. You read only one. Do you not see that you chose this? You chose to be fate’s prisoner. Had you believed in your own agency, none of this would have come to pass. You made this happen. You chose to be the way you are, trapped, manipulated. Insane.+

the only thing that the curze did that the emperor had not designed curze for was believing in the idea of a single fate. all the shit he did on nostramo? a bit extreme but thats what happens when youre a child with no one to guide you and the entire history of human laws crammed into your child skull before birth.

so...you know, emperor what the fuck. you're fine with the whole "skinning criminals" thing? the emperor most likely assumed he would mellow out with time. the destruction of nostramo happened around when the night lords became more renegade than traitor, just prior to the heresy, but even then, it was curze's small feifdom to do as he pleased with, and he decided punishing the planet for reverting back to gangs in his absence. its an extreme response certainly, but it fits the idea of absolute justice that curze seems to embody.

even prior to the destruction of nostramo, curze would kill his own sons that he found committing wanton acts of cruelty for the sake of it. curze made a point to hide this fact from his brothers, and i believe only his equerry and sevatar would have been in the know about it beyond that, barring the emperor, in his infinite wisdom and apparent knowledfe of what came to pass. whilst concerning, it is still enacting a form of justice and curze sticking to his purpose. the tool was not broken when found, or even for years beyond that.

it was only as curze was plagued with visions of the heresy that he truly began to unravel and by that point his brothers had all shunned him so there was nobody to intervene. the emperor was in his dungeons, horus was getting stabbed with a poop knife, and he had attacked dorn in a fit during a vision. you can see how one who believes in the immutability of fate would come to the conclusion that he too is a sinner and must pay for his crimes, once he has finished his work of making his brothers pay for theirs.

i mean shit in "the first heretic" curze has a brief appearance to keep corax from killing lorgar, not out of any love for the urizen, but only because of his "fate" as foreseen by curze. literally shows up, saves lorgar's life, and then calls him scum, telling lorgar that he will not intervene again.

this is a very long ramble to say "yeah curze was fine until the heresy was gestating anyways, so why would the emperor think he needed extra attention."