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.