[…] at last I knew Porphyria worshipped me […] (32-33)
When love isn't being figured in terms of freedom and restraint, it's described as "worship." Why can't love be equal in this poem? Porphyria worships the speaker (or so he assumes), so he strangles her so that he can idolize her beauty in the same way.
And I, its love, am gained instead! (55)
Porphyria has been strangled and reduced to a corpse – a mere object, rather than a thinking individual. The speaker even uses the pronoun "its" to describe her, instead of "her."
Porphyria's love: she guessed not how Her darling one wish would be heard. (56-57)
The title of the poem is almost – but not quite – uttered in line 56. It's just missing the final "r."