Nightblood Creation Theory
We know Vasher and Shasara(sp?) were attempting to create a Shardblade when they created Nightblood. I think I know why Nightblood behaves the way he does (partially)
When creating Nightblood they tried their best to picture what he was to be / do. But the intent was off. They would have understood that Shardblades were created when sapient investiture bonded with a radiant. So their goals were:
1) Pool a substantial amount of investiture 2) Give that investiture awareness 3) Create a bond between the investiture and a host 4) Destroy Evil
I think for the most part they succeeded with 1,2, and 4. But the bond is where they failed the most. Spren and Radiants have a symbiotic relationship where Nightblood is a parasite.
Nightblood gives nothing to the relationship he just wants to destroy and feed and doesn’t care that it may kill the host.
I also think the failing at recreating the Spen / Radiant bond is what causes Nightblood to make people sick. He tries to force a bond with people and create connection. By forcing himself on others he either takes over and makes them kill or he fails at connecting, leaving a person feeling deeply violated. This would also explain why he doesn’t make people feel sick after he has consumed part of a persons investiture. By having a small part of them within himself the connection no longer feels like a violation. He tricks the host into thinking he belongs.
Random musings: I find it odd that something so heavily invested with endowment can only consume / take. Endowments investiture enhances light while Nightblood deepens it. It seems like he is comprised of anti-endowment which I think is cool.