Mood is a particular response that a writer evokes in the reader through various stylistic techniques. Through the author's artistic choices, a reader can thus feel joyful, solemn, shocked, or any host of other emotions while reading.
This poem begins innocently enough, but details soon emerge that seem a bit suspect. The Duke of Ferrara, who is the speaker, points to a portrait of his former wife hanging on a wall. While some men might be annoyed by a grumbling wife, the Duke complains about his wife's joyful personality:
A heart—how shall I say?— too soon made glad,
Too easily impressed; she liked whate’er
She looked on, and her looks went everywhere.
Having a cheerful and easy-to-please personality isn't generally considered a personality flaw by most husbands, so this gives the reader a reason to carefully consider the characterization of the Duke. He continues his complaints:
She thanked men—good! but thanked
Somehow—I know not how—as if she ranked
My gift of a nine-hundred-years-old name
With anybody’s gift.
The Duke reveals that he believes his wife should have shown him greater appreciation for bestowing an old and respected name upon her; the Duchess found just as much joy in her new name and status as she did in watching a sunset or in examining the bough of a cherry tree. This was an insult to the Duke, who believes that his own "favour" should have been his wife's greatest concern and joy. This characterizes him as quite egotistical.
The Duke then reveals a shocking truth:
Oh, sir, she smiled, no doubt,
Whene’er I passed her; but who passed without
Much the same smile? This grew; I gave commands;
Then all smiles stopped together.
The Duke became enraged that his wife smiled "much the same" at him as she did for anyone else she passed. He took action, giving "commands," whereupon her smiles stopped completely.
The Duke evidently issued a command for his wife's murder. It is clear that she is dead, as he uses this poem to discuss a second marriage. The title of the poem also hints at this death; it is about his last Duchess. It is also worth noting that the Duke points out that this painting makes it look "as if [she were] alive."
The Duke's unemotional narration describing his role in the murder of his wife because of her joyful personality creates a mood of horror. The poem ends with the Duke insisting that the listener to whom this poem is addressed join him as they go to meet with the Count so that he can establish the "dowry" for his remarriage to the Count's daughter. This leaves the reader wondering what is in store for the next unfortunate woman who finds herself married to this Duke.