Tone in "Porphyria's Lover"

“Porphyria’s Lover”, by Robert Browning, uses dramatic monologue to add a dark tone to the poem. With this tone, he gives the reader an insight into the speaker’s state of mind, which turns out to be quite disturbing. At the beginning we are simply finding out about this relationship with Porphyria, and all seems well as the speaker talks about how she “glided” in and “kneeled and made the cheerless grate [b]laze up, and all the cottage warm” (lines 8 and 9). From there the speaker starts to add a little tension by saying in line 15 “When no voice replied,” by doing this he is making the reader draw in and focus on what is going to happen next. As the poem continues we make a startling discovery, he kills her,"No pain felt she; I am quite sure she felt no pain" (lines 41 and 42), but none the less the speaker’s voice and demeanor never changes; creating an eerie feeling. Additionally, the fact that the speaker seems to be telling a story, never mind the reality of him committing a murder, adds to this sinister tone.