Why are her gloves "soiled," or dirty? Sure, it's raining outside, but this is an interesting, and perhaps telling, detail to include. Perhaps the "soiled gloves" are somehow symbolic of Porphyria's "soiled" virtue or reputation.
[…] untied Her hat and let the damp hair fall, (12-13)
We pointed out the importance of the word "fall" in "Symbolism, Imagery, Wordplay" – it could suggest the Victorian idea of the "fallen woman," who has "fallen" from virtue by having sex outside of marriage.
And made her smooth white shoulder bare (17)
The "smooth white[ness]" of Porphyria's shoulder contrasts with the sin she's committing by visiting her lover.
[…] she Too weak, for all her heart's endeavour, To set its struggling passion free From pride, […] (20-24)
The speaker doesn't consider Porphyria's presence there to be a sin. For him, the real sin is her "weak[ness]" and "pride" – her unwillingness to throw off society's expectations and be with him forever.
That moment she was mine, mine, fair, Perfectly pure and good: […] (36-37)
The moment that Porphyria "worship[s]" the speaker, he decides that she's no longer committing the sin of "pride" and she becomes "perfectly pure and good."
[…] I found A thing to do[…] (37-38)
The murder of Porphyria, in the speaker's mind, isn't a sin at all – it's just "a thing to do." He's awfully casual about killing his lover.
[…] again Laughed the blue eyes without a stain. (44-45)
"Stain" is usually a metaphor for sin (like the bloodstains that Lady Macbeth can't get off her hands), but it's not clear what the metaphor of the "stain" is doing here. Is the speaker suggesting that, now that Porphyria is dead, the "stain" of her sins is gone? Or is he relieved to find that there is no visible "stain" in her eyes to mark his sin of murdering her?