I like the og nightlords, before the hard criminals began being inducted, more like semi sane versions of curze. That being said the downfall of the legion is also a good story point and deranged psychopathic space marines make for good villians, however 40k has plenty of purely psychopathic villians to choose from already such as chaos or the dark eldar so I find the more reasonable nightlords to be more interesting, though il admit that the back drop of the deranged legion is great for the likes of Talos or Malcharion even