IIRC, Index Astartes has it that the whole thing about beating up Dorn and destroying Nostromo did not cause Curze to be considered a traitor at all. Indeed, he still participated in the Great Crusade:
IA is not totally clear on this point but it seems to suggest he was still "pacifying" already-conquered worlds -- albeit in a very obsessive, disproportionate way.
As near as I can tell, the Night Lords had always done this kind of work but somehow what they were doing at this point was considered worse.
IA indicates they crossed the line by invading helpless worlds.
IMO, however, I still wonder what the difference is -- isn't the point of the Astartes that non-compliant powers would stand no chance against them? This is the question that eventually drove the Night Haunter to conclude that the Emperor was a hypocrite.
Assuming
IA is still valid, the Emperor eventually called Curze to Terra for a scolding. How exactly the Emperor was going to rein in the VIII Legion is not described -- he used the UM against the
WB and the
SW against the
TS. I wonder who he would have sent for Curze. In any case, the
HH broke out before anything could be done.
So there we have it: in the shadow of Istvaan III, Curze was not considered a traitor -- even if he was on the outs with Dorn and wanted for questioning by the Emperor.
Whether you're allowed to hold an opinion is irrelevant. Whether that opinion has any merit is the issue.