Sure, the speaker ends up killing Porphyria, but the poem includes "lover" in the title, so you have to figure that "love" is going to play some kind of role. You're right, though what passes for "love" in the world of this poem isn't going to win you any prom dates. After reading this poem, you'll likely feel that the speaker has earned a one-way trip to a federal prison. Or to a mental hospital.
In "Porphyria's Lover," love is always figured in terms of freedom and restraint, suggesting that, ultimately, neither Porphyria nor the speaker have any real agency.
Porphyria's love for the speaker is described as "worship," while his love for her is violent and objectifying. In the end, both kinds of love alienate the subject from the beloved.
There's some pretty obvious sinning going on in "Porphyria's Lover" – after all, the speaker describes how he strangled his lover. But there's some less obvious sin here, too. Victorian moralists were all about repressing female sexuality and pretending that it didn't exist. For a woman to acknowledge that she even had sexual desires was considered sinful, and actually acting on those desires was borderline criminal. So for Porphyria to "come through wind and rain" to be with her lover was seriously risqué (line 30).
Within the world of "Porphyria's Lover," murder is not condemned; it's only by applying an external system of right and wrong that we're able to gauge the crimes of the speaker.
The final line of the "Porphyria's Lover" teases the reader by flaunting its absence of an ethical system: the only system available in this poem is one that is imposed by the reader.
The "love" between the speaker and Porphyria turns pretty quickly into a power play. Porphyria seems to be the one who's in control at the beginning of the poem, then the speaker completely reverses things. He seems to want to possess Porphyria, so he reduces her to an object (a corpse, instead of an independently-thinking individual).
The speaker of "Porphyria's Lover" murders Porphyria in order to reduce her to an object that can be controlled and admired.
The speaker's power over language breaks down at several key moments in the poem, suggesting that his calm control is only a superficial veneer.
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.
This is one of the more understated themes of this poem. Porphyria seems to be of a higher social class than the speaker. Her reluctance to be with him might have to do with her reluctance to give up social standing. Death, however, acts as a social leveler – killing her makes her social class irrelevant.
The difference in social class between Porphyria and her lover creates a disparity in their power dynamic during the first half of the poem. The speaker murders her in an attempt to bring balance to their relationship.
The difference in social class between Porphyria and her lover makes absolutely no difference: their power dynamic is uneven because of her intense agency and his extreme passivity, not because of her social rank.