How does the duke describe Fra Pandolf's activities in "My Last Duchess" and why is this important?

As the artist, Fra Pandolf, paints the Duchess, the Duchess flushes (blushes), which is apparent in the artist's rendering, the portrait, and Pandolf may have said that his work could not truly capture the beauty of the Duchess. However, the Duke has no evidence that the painter flirted with the Duchess;the Duke is insanely jealous and arrogant, and it is this arrogance that, more than likely, leads to the death of the Duchess. The Duke also believes the Duchess flirts with every man: "Her looks went everywhere." Unfortunately,a good artist must capture the personality of his subject, and in this case, ironically, by capturing "the depth and passion" of the Duchess, Fra Pandolf contributes to the Duke's unavoidable jealousy. Thus, paradoxically,the Duchess is immortalized and owned by the Duke as art. At the close of this poem, the Duke elevates himself to become, metaphorically, Neptune, and the poor Duchess, a sea-horse: "Notice Neptune, though, taming a sea-horse."