This is situational irony
situational irony is when something is expected to happen but the something else occurs. In this case it is Romeo try to win over Rosaline, but then falls for Juliet.
Answer:The speaker believes that Porphyria herself wants to die because he says she felt no pain. In the end, the speaker says, 'And yet, God has not said a word!,' which probably means that he felt God would punish him for the murder. In this interpretation, the themes are Love and Sin.