Browning's "Porphyria's Lover" is as much about power as it is love. He takes a narrative about an illicit affair and does two things with it: makes it beautiful and makes it about power. When the speaker strangles Porphyria, he reverses who is in control of the relationship.
Notice that the speaker is a literal and figurative mess at the beginning of the poem. It is Porphyria who enters and takes control. She starts the fire and cleans up the cottage and takes care of him. She places his head on her shoulder. This represents a reversal in normal Victorian gender roles. Porphyria controls every aspect of the relationship, including when the lovers meet. She comes to him when she can and when she chooses to. The male has no control.
The speaker's strangulation of his lover reverses the power structure in the relationship. He rests her head on his shoulder, once he is in control.
The narrator is, of course, unreliable. He interprets the look on her face as meaning that she has finally given herself totally to him, and to preserve that moment, he freezes it by killing her. His insistence that she felt no pain is the most obvious evidence of his unreliability. The speaker is insane. The poem is primarily about his insanity--dramatic monologues are always about characterizing the speaker. He sees what he wants to see; he takes control when he wants to take control. And he talks about it with a truly silent listener--a dead one.