In Robert Browning's poem, "My Last Duchess," the speaker describes his last wife—a lovely woman to whom he was once married—while exposing how arrogant and dangerous he is.
The man in question is the Duke Ferrara. He is showing a lovely portrait of his "late" wife. Note that it does not say my "first" duchess, but my "last." The Duke is speaking to a member of his future fiancée’s family. He identifies who painted the picture, and then shares the fact that no one sees this painting unless he wishes it so. And when he opens the curtain, her gaze looks only at him:
The depth and passion of its earnest glance,
But to myself they turned (since none puts by
The curtain I have drawn for you, but I)...
Even after her death, he seems jealous of her attention.
He notes that the look on her face was easily come by...it was not, unfortunately for her, a look that she guarded only for her husband. A casual remark by the artist that made note of her "mantle" falling too far down on her wrist, though only "courtesy," would make her blush with "that spot of joy." The Duke goes on to note that this was a failing:
A heart---how shall I say?---too soon made glad,
Too easily impressed; she liked whate'er
She looked on…
This does not mean she flirted or was unfaithful: she was simply very easy to please and found joy in countless things. Mere jealousy does not convey the Duke's depth of feeling; he catalogs her many insufficiencies. Upon reading the poem, one is left to wonder if the speaker is not just a pompous fool. However, further reading makes the reader wonder if he is not simply a dangerous and insane man.
She found pleasure, he notes, in a sunset, "[t]he bough of cherries some officious fool / Broke in the orchard for her," and even the white mule she rode on around the estate. His censorship is clearly tied to his enormous ego and sense of entitlement. He points out that he deigned to bestow upon her his "nine-hundred-years-old name," and she thanked people as if anything they could do could measure up to his social standing—which he believes impossible.
Even though he thinks on the things she does with displeasure, he never says anything to her. Consider that he wanted to say he found things...
…that in you disgusts me...
However, he declares he would not "stoop"—that having to explain himself was beneath him.
E'en then would be some stooping; and I choose
Never to stoop.
He could not control her in life. The speaker notes that she smiled as she passed him, as everyone did, but her smiles were not only for him alone. And so, he insinuates that he took steps to stop her behavior. However, make note of several things: he gave commands; "all smiles stopped together;" and, he notes that "...she stands / As if alive." Her smiles stopped after he gave commands. And she is now dead, as he prepares to marry again. It's possible she died of a broken heart. However, the Duke hardly seems a man who would wait that long. His "commands" may well have assured her death (perhaps by "assassination"). He seems mad to go about speaking of it so casually—but he is demonstrating his power.
In the last few lines, the Duke further emphasizes his supremacy by making a comparison related to Neptune dominating a seahorse:
The image of the powerful god taking control over a creature like a sea-horse demonstrates the relationship between the Duke...and the last Duchess.
This also sends a message to his future bride.