The second theme is this: Shocking, unexpected behavior is part of life.
Not infrequently, seemingly normal and harmless people turn out to be child molesters, rapists, serial killers, and so on. Porphyria was not identified and named as a disease until 1874.
In this case, it is nighttime, and the thunder is roaring.
The speaker starts by saying: "The rain set early in tonight,/The sullen wind was soon awake,/ It tore the elm-tops down for spite,/ And did its worst to vex the lake(Barnet 567):" This description gives the reader the first glimpse of what is yet to come.
The speaker is a deranged man who will stop at nothing to keep his dear Porphyria.
Although the introduction refers to the weather, it also does an effective job in describing the speaker.
As the rest of the poem is written in first person a line written in the third person such as "when no voice replied" shows that the persona is not himself as there may be some insanity involved, obsession has consumed him and he has no control over what he is doing.
"Mine, mine" - repetition that emphasises the emotional triumph and possessiveness for Porphyria, his Obsession.
It is the story of a man who is so obsessed with Porphyria that he decides to keep her for himself.
The only way he feels he can keep her, though, is by killing her.