I think one of the big reasons the other Primarchs dislike Curze (outside of his legions tactics and his ideals around justice) is that Curze's work at an unconscious level reveals to them how actual horrid and cruel their whole Great Crusade really is.
Of course they will never say it as such, but that's the reality of it. Most of the Primarchs hide behind the actions they do as either a fault of the people they are trying to conquer for fighting back/disagreeing, loyalty to the Emperor, a necessary evil, or as the "right thing" because the Imperium brings the truth and enlightenment.
It's all easy to say you are doing this colonization, mass murder, and in certain cases, genocide for the betterment of humanity as a whole when your rose tented glasses aren't broken by a guy who is sanctioned to do the exact same thing but in a way you view as inhumane and deplorable.
Because its not about the people being hurt, not really. The Great Crusade was one giant gilded excuse of mass harm against people for no good reason.
Konrad just made it difficult for them to look away and say it was all necessary.
