Three connections between three of Robert Browning's poems

My Last Duchess, Porphyria's Lover and The Laboratory.

The first is set in Ferrara, Italy and is a dramatic monologue about a Duke remembering his old wife who he had killed, the second is set in the eponimous lover's house and is a dramatic monologue about a Man killing his lover and the third is set in 17th century France and is a dramatic monologue about a woman killing her rival in love. See the main connection? All three of these poems are told as DRAMATIC MONOLOGUES (i'll get to the murders in paragraph two, OK, be patient). A dramatic monologue is a poem told exclusively by one character and all descriptions of scenery and other characters come from the mouth of that character. Writing poems in this form, Browning makes us readers try to sympathize with these thoroughly unpleasant characters. If told in second or third person, the main characters in these poems wouldn't have a hope in hell of getting any sympathy, but told in the first person like this, it takes you longer to see through their lies.

These particular three poems are also connected by the way in which they all center around a murder having been committed, being committed and yet to be committed. In My Last Duchess, the Duke has murdered his wife BEFORE the poem, in Porphyria's lover, Porphyria's lover murders her DURING the poem and in The Laboratory, the poisoner lady murders her rival AFTER the poem. So, even though all three poems concern killing, they all take it from a different place in the timeline. The preparation (The Laboratory), the act (Porphyria's lover) and the aftermath/dealing with conscience (My last Duchess). This devise, the crux on murder, succeeds in making each poem feel similar to the last, but not exactly the same.

My Last Duchess uses regular Iambic Pentameter ( tum TE tum TE tum TE tum TE tum TE).
The Laboratory uses it too.
Unfortunately, Porphyria's lover just had to ruin everything by having four beats per line (tum TE tum TE tum TE tum TE).
Iambic pentameter is the form that comes naturally to us, as it sounds the most natural and flowing. Robert Browning uses this form in My Last Duchess and The Laboratory to show us that the characters think what they're doing is perfectly normal. He uses four beats in Porphyria's lover to show us that the Lover is scared and not at all sure that what he has done (Murdered Porphyria) was right. He even has to reassure himself in line 42 "I am quite sure she felt no pain". He had just said words to that affect in line 41. This proves that, the Lover, unlike the Duke and the Poisoner felt remorse.