Personally (and this is based only on my gut feelings, which have been wrong many times in the past) I think NK's behavior, like spilling the beans and how she was acting in the booking video, shows that she is either completely stupid or believes herself to be quite a bit less culpable than DE. I'm in the camp that thinks DE is the mastermind behind it all, and both NK and NL were easily manipulated and overly trusting. He definitely seems charismatic enough to have manipulated both these girls. His arrogance, his speech patterns (not an expert on this, but the wooden way he speaks is very creepy to me) and his whole demeanor seems sociopathic to me. The murder may have been sexually motivated, but I kind of think it might have just been a thrill kill too.
Feel free to ignore this completely though, it's just my, admittedly non-expert, opinion