The tone of the poem is that of an ignorant, vulgar, insensitive, selfish, arrogant, and brutal man who is trying his best to sound like a courtly aristocrat. One of the ways in which this tone is achieved is through the content of the poem itself. The speaker inadvertently reveals all of his many faults while describing his lovely "last duchess" whom the visitor can see right in front of him. What would seem to most people to be good qualities in the young woman seemed offensive to the Duke, although he couldn't explain exactly why this was the case. He finally reveals that he apparently had the poor girl murdered and that he is now looking for a replacement who will bring him some more money as her dowry.
The other way in which Browning establishes the rather repellent tone of the poem is through the use of open rhyming couplets which are so awkward, ragged and crude that they betray the speaker's low intelligence and vulgarity, in spite of the fact that he pretends to have refined taste in art and lives in luxury. For example, here are the opening lines of the poem:
That's my last Duchess painted on the wall,
Looking as if she were alive. I call
That piece a wonder, now: Fra Pandolf s hands
Worked busily a day, and there she stands.
Will't please you sit and look at her? I said
"Fra Pandolf" by design, for never read
Strangers like you that pictured countenance,
The depth and passion of its earnest glance,
Browning, through the Duke, rhymes "wall" and "call," "hands' and "stands," "said" and "read," and so forth. Typically the word at the end of one line runs right into the next line, so the reader may well wonder where to pause to take a breath. This unusual feature also gives the impression that the Duke is a man who is used to doing all the talking and having everyone else do all the listening, a sign of his arrogance and despotism.
The Duke is rudely interrupted when the visitor can stand no more of this brutal man or his memoir and abruptly leaps up and starts to hurry down the stairs. We are happy to see the Duke discomfited and deprived of the opportunity to discuss the dowry of the young girl he hopes to marry.
Will't please you rise? We'll meet
The company below, then. I repeat,
The Count your master's known munificence
Is ample warrant that no just pretence
Of mine for dowry will be disallowed;
Though his fair daughter's self, as I avowed
At starting, is my object. Nay, we'll go
Together down, sir.