If Nightblood were to cut one of the Evil how would it go? They are both heavily invested entities that consume all the investiture they touch. I think that there are definitely some intentional similarities between the two of them.
A different kinda unrelated theory but could the reason why the reason why both The Evil and Nightblood have similar attributes be because of some cognitive idea of Evil? Like there is some sort of association of specifically consuming massive amounts of investiture and evil that could have effected both the intent of Nightblood's command and how mental stuff affects The Evil? (This might very much be a stretch)