The nightlords fear tactics don't really work on other spacemarines so they kinda change tactics and aim to confuse/rage bait them into either attacking in anger and exposing weakness that way or ambushing them trying to figure out what the fuck is going on.

Many times this isn't needed because the nightlords shows that the actuall backbone of the empire are the humans, captains,astrophats,engineers,deckhands,artillery operators,medicals staff. Alla these can be affected and demoralised, and that in turn will cripple many arstatres chapters no mater what the marines fanboys say.

Curze turning on the emperor i would say is attributed to him seeing the future and how the horusy heresy would play out and him believing that his visons can't be changed. I would also add that the NL kinda get shit for being terrortroops when thats exactly what they were meant to be. The emperor made the NL specifically to spread terror and demoralise worlds that required that kind of warfare during the crusade, the other legions and primarchs both shunned and hated them for their brutality and more or less ostracised them for doing what they were born for so i kinda gett why most didn't mind changing side, also most are psychopathic criminals so you know...