Porphyria and the speaker keep switching places. At the beginning, the speaker is passive, and allows Porphyria to move his arms around as she sees fit. She does everything, while he just sits on the couch like a lump. But then, abruptly, they swap: the speaker strangles her, and makes Porphyria even more passive than he was.
The active gaze of the speaker, as he turns to Porphyria in line 31, marks the moment in which agency passes to him. This change suggests the dehumanizing power of the male gaze in the world of "Porphyria's Lover."
In the first half of the poem, Porphyria demonstrates a kind of intense agency that the speaker can't accept: her agency and his passivity are so extreme that the only way he sees to reverse their roles is to murder her.