"Porphyria's Lover" was quite subversive for its time (and to an extent, still is) but prose writers of Browning's Victorian England were also dabbling in Gothic literature and horror. This is one of many examples where Browning shared more with his contemporary prose writers than with his contemporary poets.
The poem is a dramatic monologue which means the speaker addresses someone (perhaps himself, God, the reader, or some other) and his words and thoughts indicate to the reader his character and/or state of mind. The ababb rhyme scheme and occasional enjambment (lines which grammatically carry over from one line to the next) establish a subtly odd phrasing which parallels the subtle ways Browning establishes the state of mind of the speaker (we get subtle clues but are taken by surprise with the murder).
The poem is about the speaker murdering his lover, Porphyria, by strangling her with her own hair. This poem is an exercise in considering madness, the potential link between violence and sex, and the psychological impact love can have (in this case, on an insane speaker; however, the reader is also left to wonder if the speaker is not insane, perhaps merely a liar).
The calm, casual way the speaker describes the murder is strange, reflecting the warped mind of the speaker. And the event of the murder seems to come out of nowhere unless we consider that the murder is a shift of dominance. When Porphyria comes in, she is active and the speaker is passive.
She put my arm about her waist,
And made her smooth white shoulder bare,
And all her yellow hair displaced,
And, stooping, made my cheek lie there, (16-19).
Notice that she physically controls his movements and "makes" his cheek lie. The speaker, mad with love and insecurity, sees a moment where he can become the dominant figure in their love and takes it, and this takes the reader by surprise. He waits until the "moment she was mine, mine, fair,/Perfectly pure and good." Therefore, he can be with her in this so called "perfect" state forever.
"Porphyria's Lover" is similar to Poe in its treatment of Gothic subjects. And some critics claim that a full analysis of this poem along the lines of Gothic horror has been overlooked. Check the third link below for an analysis which posits that the speaker is not really insane; he kills Porphyria believing she is a vampire. This interpretation is a bit of a stretch, but horror was a contemporary subject in Browning's time. For example,Frankensteinwas published in 1818, Poe lived from 1809-1849, and "Porphyria's Lover" first appeared in 1836.