These are apparently two women who had the men in their lives so afraid of losing their love that they killed to keep them where they wanted them, to control them or at least to control the moment, whether it was in the frame of a portrait or on the bed or couch that the two lovers last shared with one another. One of them even sat up and waited all night for God to come get him for the sin and the crime. If Porphyria's lover was waiting for God to react to his selfish deed with punishment of some kind, (presumably with a bolt of lightning strategically placed to his left temple for the commission of the act of murder) why did he not just go ahead and kill himself, too
The main overall theme of both Robert Browning poems "My Last Duchess" (hereafter known as "Duchess") and "Porphyria's Lover" (hereafter known as "Lover") is jealousy; as in jealousy mixed with contempt for beautiful women, all the way to the point of literal physical death.
The relationship between Porphyria and her lover is positive and has a strong bond. This is why it appears that he (the Lover) could not deal with the thought of being without her. In "Duchess," the woman that is the target of the Duke's affection (worship nearly) has already passed on and all he has left of her is the painting on the wall, to which he, the narrator of the poem, refers at the very beginning.
In "Lover," the script is flipped. It is the object of the speaker's discussion (the lady Porphyria) who nearly worships her man instead. He says this directly. (Browning, ll's 32 and 33 "Happy and proud; at last I knew Porphyria worshipp'd me"). It is with these 10 words that the narrator of the story tells us that he realizes how far she had come to tell him of her love, and he then reveals to us, the reader, how far he must go to leave things where they are. The only clue that we have of a possible motive is when he says in lines 21-25 that she would never "give herself" to him "forever." For what it was worth and for all the love she professes for him, it appears that she came to tell him she could not loosen the grip that societal mores had on her just to be with him.
There is no indication that Porphyria is married, or that the adored one, the narrator, is beneath her upscale standards, so we never find out exactly why Porphyria did not want to 'rock the boat,' in so many words, to be with this man that she obviously adored beyond compare. There could have been many reasons in the Victorian age, or the age of prudishness, cloaked over as modesty and righteousness. Unlike the Duchess, who is dead from the start, Porphyria is not referred to in the sense of a decedent until near the end of the story when the narrator's madness is revealed (Browning, ll's 39-42, "In one long yellow string I wound Three times her little throat around, And strangled her. No pain felt she; I am quite sure she felt no pain) It appears that she didn't even put up a fight. Why did she not cry out or fight or struggle Is this some indication that she would rather die rather than be without the man of her dreams
The Duke in "Duchess" also speaks of the woman's throat (the "faint half-flush that dies along her