Sparteye is correct!
In spite of the def.'s listed by tsuwm, UNrighteous is not the the same as NOT righteous. Using the number line again as a ref., unrighteous carries the implication that the action was confined to the negative side only: that is, actively "evil; wicked; sinful".
Not righteous, to my way of thinking, can signify behavior that is at the zero mark, or neutral--not actively good, NOR
actively bad.
I don't think the term not righteous is called for very often, but it has its place, and should not be equated with unrighteous.

EDIT: Well, shanks, you snuck in and posted ahead of me.
Yer gonna have to show me how my "plank" is "fallacious".