That moment she was mine, mine, fair, Perfectly pure and good: […]
In this moment when he looks up at her eyes and realizes that "Porphyria worshipped" him, the speaker decides that she's completely his.
He repeats the word "mine" twice, in fact, to emphasize his feeling of possession.
Everything about her, and about this moment, is "perfect."
[…] I found A thing to do, and all her hair In one long yellow string I wound Three times her little throat around, And strangled her. […]
What do you do with a moment that's so "perfect"? Soak it up and enjoy it so that you'll remember it forever? Take a photo? Not if you're the speaker of "Porphyria's Lover."
He figures out what to do – he takes her hair and twists it into one "long yellow string."
He then wraps the "string" around her throat and strangles her.
Wait, what? Did we just read that right? But the poem sounds so musical and light! He wraps it "three times" around her throat. It sounds almost like a fairy tale or a nursery rhyme. What's up with this guy?
[…] No pain felt she; I am quite sure she felt no pain. As a shut bud that holds a bee, I warily oped her lids: again Laughed the blue eyes without a stain.
Don't worry, though: the speaker assures us that Porphyria didn't feel any pain. He's certain of it.
The speaker then carefully opens ("oped") Porphyria's eyelids.
He compares this to opening a flower bud that might enclose a bee. (Go to "Symbolism, Imagery, Wordplay" for more on that weird metaphor.)
Once he gets her eyes open again, Porphyria's pretty blue peepers "laugh" again.
Creepy! How can a dead woman's eyes "laugh"? Obviously the speaker is out of his gourd. And if he's assuming he can tell that her eyes are "laugh[ing]" now, should we believe what he said earlier, about Porphyria's eyes looking "happy and proud" (line 32)? Do we need to rethink everything the speaker has told us?
He also says that her eyes are now "without a stain." What's that about? Is he referring to a metaphorical "stain" on her honor? Or does the metaphor of the "stain" refer to her unwillingness to be with him exclusively? It's not clear. But somehow, now that she's dead, there's no more stain.