How does the mood of the poem change over the course of Browning's Porphyria's Lover?

Porphyria's Lover is typical of Robert Browning's dramatic
monologues in that the poems reveal as much about the dramatic narrators as about the
putative subjects of the poem. Several other poems by Browning, of which the best known
is probably "My Last Duchess", start with what appears to be a narrator describing his
love for a woman. The opening of "Porphyria's Lover" is foreboding, with the weather
mirroring the narrator's mood of despair. When Porphyria first appears, the mood changes
to tender and romantic:

When glided in Porphyria; straight

She shut the cold out and the storm,
And kneeled and made the
cheerless grate
Blaze up, and all the cottage
warm;

but the lover's jealousy eventually prevails, and the
violence foreshadowed by the opening surfaces:

Made my
heart swell, and still it grew
While I debated what to do.
That
moment she was mine, mine, fair,
Perfectly pure and good: I found
A
thing to do, and all her hair
In one long yellow string l wound

Three times her little throat around,
And strangled
her.

The mood of peace at the end, with the narrator
sitting with the corpse of Porphyria and the voice of God (and perhaps weather) quiet,
adds an eerie twist to the poem.