If the denotation of the word "porphyria" tells of an imbalance of the hormones,then this monologue tells about it's connotation.
First we see that "The rain set early in tonight", implying that the man sense the "storming" of porphyria coming earlier than he had assumed.He wanted to meet his lover,but the troublesome porphyria is stepping on his way.
The sullen wind was soon awake,
It tore the elm-tops down for spite,
And did its worst to vex the lake
I listened with heart fit to break.
The four lines above-mentioned is suggesting of what will happen later between the man and his mistress:"the sullen wind" is already telling the readers of the mournful scene we might encounter later;"the elm-tops torn for spite" shows the insanity that had its possession over the man,making him commit crime scenes.The storm is already brewing,only to wake the perfect moment to spread it around.
When glided in Porphyria;straight
The mistress,the one our speaker loves,came in at this very instant,and this is just the moment when the struggle of the man with the insanity has begun.
She shut the cold out and the storm,
And kneeled and made the cheerless grate
Blaze up,and all the cottage warm;
The "She" in here could mean the mistress,whom the arrival has brought contentment to her lover,and also refers to "porphyria",the disease that troubles the speaker so much,but he wasn't completely aware of this.
Now here comes the seductive scene,both from the mistress,who is trying to pour her love toward the man through body language and words;and porphyria,which slowly pours its venomous poison onto the speaker,driving him mad step by step;and as the passion goes higher and higher,like in those thriller movies,where seduction reaches its highest peak,the irrational comes out,consuming the essence of passionate loving ones.Through the seduction of the mistress toward the speaker,whom we assume should be of very lustful,yet tendersome,the speaker himself didn't see his lover as we see her.He's,at this point,completely possesed over by porphyria,which has eaten his soul,so that he thought of his mistress too weak to love him.He was to possess her love totally and forever,therefore,a sudden thought had passed by his mind,and this is the moment when I felt the chill on my back.He was going to do it!He thought that love means totally possesing someone,and as he looked at his mistress in her utmost beauty(when she has given him,totally),he wanted to keep it that way forever,so he killed her,by strangling. But it was not killing for him,it was restoring the best side of his mistress for him,save the best forever.This is the highlight where the speaker has lost his mind completely,yet he felt "the unconditional love" of his mistress as well.
A perfect murder expressed through a perfect monologue---I feel like throwing up!