I agree, Kurze was not mustache-twirling evil, but he was a sociopathic murderer. For anyone who's seen the show Dexter, there's a parallel you can draw:
Kurze grew up not so much knowing right from wrong as knowing order from chaos. He did what needed to be done to instill order on his planet. Every primarch seems to have had this as a basic instinct. This is his "code" that he lives by - using any means necessary to maintain order and obedience.
Dexter is a sociopath as well, not inherently knowing right from wrong, but having his father carefully teach him how to fit into society and who it's ok to kill (bad guys). He has learned a code and he sticks to it rigorously even if it doesn't come from any internal sense of right and wrong.
The Emperor sees the value in Kurze's abilities and uses him as a tool to keep conquered planets in line (the same way Dexter's dad essentially used Dexter as a tool to kill people who deserved it but escaped justice). There is a thin line being walked - very dangerous characters who kill and torture without remorse, but who maintain self-control and apply themselves to what they see as a higher calling. Kurze is constantly tortured by dark visions and desires that he struggles to keep in check.
Now, imagine if Dexter's dad came back and told Dexter that everything he thought he knew about how to live life was wrong. If Dexter's dad basically revealed he had been using his son to settle old scores, and now no longer needed him to kill people. Dexter's whole internalized code that kept his killing in check would basically disappear overnight, and he would probably become quite unhinged in the process. Who is he allowed to kill? Who deserves to die? Does his own father deserve to die for what he has done?
This is essentially what happened to Kurze. Once the other primarchs started to realize he was a dangerous nutter, and after whipping Dorn like a red-headed stepchild, Kurze was basically cast out by the only authority figure he knew, the Emperor. Sent to punish wayward planets as only he could, then told he was wrong to do so? Kurze saw hypocrisy in the Emperor's actions - unable to see right and wrong for himself, he had attempted to follow a black and white worldview where order was desirable at all costs, and now the Emperor had reversed himself, shamed Kurze, and proved himself arbitrary and self-serving.
Unable to see any shades of grey in terms of morality, Kurze doubles down on what he thinks he knows - order at all costs - and decides for himself the Emperor is a liar, a weakling, lacking conviction to do what is needed for the Imperium to work. He's no different from the corrupt politicians and crime-bosses that Kurze took down on Nostramo, and he
used Kurze for his own purposes. Here Kurze is especially interesting because even without his visions of dark futures, he sees something about the Emperor that his brothers don't - that regardless of the Emperor's intentions, in the end he cannot maintain order in the Imperium. As a result the Imperium is weak, rotten, doomed to fail.
Now after all that, Kurze returns to find his own planet has descended into lawlessness again. This is the point where I think Kurze really loses it. Nostramo is like a microcosm of the Imperium, proving everything will always decay into corruption and lawlessness. The punishment must be death. What existed of Kurze's moral code has fallen apart - the only authority figure he ever knew has proved to be a sham, and even his own planet cannot be kept in line. The whole planet needs to be put down. The whole
Imperium needs to be put down. The only true authority, the only true code, is unbridled cruelty and murder, and there is no-one left in the galaxy who does not deserve it. At this point Kurze is a rabid dog who has slipped his leash, with nothing whatsoever left to keep his murderous urges in check.
So, yeah, fun character. I'll get off my soapbox now