Find your tribe in a Sea of Creativity
So, I just had a probably crazy idea.
What if Nightblood isn't actually completely native to Nalthis? What if Nale is bringing it back home when he gives it to Szeth? Nightblood could have been a Rosharian Shardblade (Szeth doesn't seem to question it being a shardblade) that ended up on Nalthis. Then when Vashar and Shashara awakened it, the process roused the dead/trapped spren in the shardblade. From what we have seen, spren require an active bond with a human to remain sentient in the physical realm. Awakening would interact strangely with this resulting in the spren needing a steady supply of breath to remain active. This could provide an explanation for why it sucks the breath out of people (something that is never really explained in Warbreaker).
Nightblood's inability to fully understand what evil is and place it in a human context runs reasonably parallel to Pattern and Syl's attempts to understand the more abstract concepts of the human world. In Nightblood's case, the spren is still trapped in the sword and since it doesn't have a proper bond with a person, it is even harder for it to learn, so it can't progress as well or as quickly as Pattern and Syl.
The more I think about this the better it is working...