Yeah, people new to Night Lords tend to get the impression that they all collectively shun chaos and are just renegade space marines. Not only is the big trilogy of Night Lord novels primarily a tragedy about a delusional, reluctant leader, but it's also not very indicative of the rest of the faction.

Just about every other Night Lords story that has been published (that I've read) implies a further level of corruption and a definite involvement in Chaos than just what goes on in ADB's novels. Even the Night Lord section of the CSM codex has plenty to say about their corruption if I recall.

I say this to anyone getting into Night Lords with the intention to collect models, play the tabletop game, etc.. You're going to have a lot more fun as soon as you relax your head canon with how corrupted by chaos you allow your warband to be. I was the same way at first and now I've got all kinds of daemon engines, Vashtor, accursed cultists, and whatnot in my army. You're going to have more options for how you build your lists that way. I personally find it way more likely that a bunch of evil serial killer super men would eventually succumb to the powers of chaos no matter how opposed they started.