Sorry but thats a straw argument. Obviously everyone dies when killed but what you are saying is equal to a man armed with a knife having a chance against a machinegun nest on a frontal asault. Despite its obvious fantasy themes, the story tries to be realistic so you dont get ubber assasins that sneak upon dozens of guards and kill his target as easily as slicing a bread. Even if by some kind of miracle the assasin manages to get his kill theres now way that Nanamo will escape unharmed since you cant even take a piss in Uldah without the Syndicate knowing it, and they could easily choose to answer blood for blood or just judge her for murder and get rid of her cleanly and theres NOTHING that Nanamo could do against it. Being realistic the instant that Nanamo tries to do something funny the Syndicate will know it FAST, as saw in some scenes where it was obvious that they are under constant vigilance