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.