With any work of literature there is a multiplicity of possible themes that can be drawn out. However, examining this poem, the main message or theme that appears to me is that of pride. As the narrator of the poem is the Duke himself who, we discover, was so jealous of his former wife and the attention that he perceived she gave to others, that he "gave commands" (indicating that he had her put to death), we see everything through his eyes, and we must piece together his character. So sure of his own situation and position that he happily narrates this tale of his "last Duchess" to the very emissary who is organising his next marriage, almost as if it is a warning of the kind of behaviour he expects and demands from a wife. When we think of this, we begin to see this narrative in a different way. Really, the Duke is using this story to outline the conditions of his future marriage and the kind of submission that he feels he deserves from his next wife. Note what he says of his last wife:
--and if she let
Herself be lessoned so, nor plainly set
Her wits to yours, forsooth, and made excuse,
--E'en then would be some stooping; and I choose
Never to stoop.
Note what these lines reveal about the Duke. He is obviously a proud man who considers it beneath his position and dignity to discuss with the Duchess his concerns about her reactions to other men. He clearly believes that his wants should be anticipated without the need for him to "stoop" to ask for anything. Obviously this presents the theme of pride, as we are struck by the arrogance of the Duke and the way he both reacted to his "last Duchess" and his demands of total compliance of his next wife.