Although there are many themes in each work of literature, if I had to boil this down to one, I would say that Robert Browning stresses the theme of violent love (especially in relation to madness) in his poem "Porphyria's Lover."
Upon first glance, the idea of "violent love" seems like an oxymoron; however, after reading the poem, the relationship becomes clear. The speaker has a desire to keep Porphyria all his own. In order to do this, he strangles her with her own hair.
There is no doubt, however, that love (or at least infatuation) exists between the two:
She put my arm about her waist,
And made her smooth white shoulder bare,
And all her yellow hair displaced,
And, stooping, made my cheek lie there,
And spread, o'er all, her yellow hair,
Murmuring how she loved me
According to this quotation, the love between the two people here is quite passionate and sexual in nature. Unfortunately, the extreme passion that exists between the two leads the speaker to violence in order to claim Porphyria for his own, forever:
A thing to do, and all her hair
In one long yellow string I wound
Three times her little throat around,
And strangled her.
As love leads to violence, one has to admit that mental illness, or if I may use the more common term "madness," to be involved here. The speaker talks about how Porphyria's blue eyes "laughed" and how her head was still "smiling" after she was dead. Further, the act of murder itself caused her to feel "no pain.' The speaker strangles her as he is "quite sure she felt no pain." He then props her "laughing" eyes open yet again, to kiss Porphyria as she lay there, dead. However, probably the biggest indicator of madness can be found in the last few lines:
And thus we sit together now,
And all night long we have not stirred,
And yet God has not said a word!
The speaker is now sitting, hour upon hour, and caressing a dead body. Death has not seemed to stop the speaker's sexual attraction to Porphyria. Further, there seems to be an indication here that the speaker's grave sin would have actually prompted God to speak. Thus, the speaker sits, clinging to a dead body for sexual gratification as he waits for God to rebuke his action. Yuck!