He is neither skaven (a ratman chaos beast) nor an actual assassin (a murderer of an important person in a surprise attack for political or religious reasons), as he failed to actually kill Nagash.
But as you did rest your case I will have to assume I have won this debate!
Yes, I'm a ninja. No, I won't show you my moves.
I do not sleep, I wait.... It's a ninja thing, you wouldn't understand.