What's the story with the Night Lords than? I assume they've fallen into chaos, but why are they so opposed to the dark gods of the warp? Is it through a scepticism of their existence or more a scepticism of their power? My WH46K is limited to wiki articles and the games, but I've pursued the wiki quite thoroughly, and find myself loving the universe more than I imagined so far.

I understand the reader/player perspective draw of the Night Lords, a group of Chaos Marines whose turn on the Emperor was more out of principle than the allure of power (Or so I've interpreted thus far) but it seems slightly odd in the lore perspective to me that they'd be not only undevoted but sceptical by virtue of their very legion. It just seems to me quite a few 'corrupted' and 'tainted' designs are important to the Chaos Space Marines now, such as in their architecture, vehicle design and unit rank/power progression, do they have a way of getting around these things or are they more relaxed when using daemons to their own advantage.

Also I guess you should be careful about spoilers, I may not read the series but others here might.