I'm pretty certain it wasn't that difficult to realize Orange had a unique role in game, he did basically tell everyone. Considering that he does have a unique role, that makes him more of a threat than other roles, and as such a higher priority to kill than a vanilla role.
In the absence of a better target, he's a really good target for the NK to make certain you get rid of a annoying role you'd have to deal with later on. And there's no guaranteed that he was telling the truth about his night action either, and he had made up his claim to hide his real ability.
He may not be the best NK target to go after, but he's still probably a better NK target than randomly trying to pick from the remaining players who has an actual power role.