Kemen doesn't have aptitude to be one of the Nazgûl. It was said that Nazgûl were from the mightiest or most powerful of men.. I believe they were kings, knights or valiant nobles, or powerful witches (ancient english Tolkien used have whatever word that derived witch as gender neutral). Kemen definitely not one of them. He deserves to be stomped by ents and eaten by orc, not glorious death by Eowyn or cool track record as Witch-king.