My question would be if the Duke is a figure who's purpose in the narrative is to be an assassination target then why would he ever be anything BUT a minion?
Well, it would depend. In the example I gave, the Duke might not be explicitly an assassination target: he might be someone that the PCs get into a dramatic swashbuckling swordfight with in the middle of his burning tower. The assassination would be an alternate way of dealing with That Kind of Guy, rather than the default.
But if the point of the NPC is to be assassinated, then yeah. Why not make him a minion from the get-go?