How does Robert Browning use imagery in Porphyria's Lover?

Browning seems to take pains to make the musings of a criminal psychopath clearly understandable to every reader. The poem uses simple, short words. However, there are subtle developments in the poem to suggest the speaker's unusual state of mind and his heightening sense of conflict. At first, the poem relies almost exclusively on straightforward description as the speaker recounts the events that have taken place, but as it becomes clear that the events described are seen through the lens of the speaker's madness, the language becomes more metaphorical. In the early description of Porphyria, the speaker offers a simple physical description of her. She has smooth shoulders and yellow hair. But after he kills her, he uses vegetative imagery to describe her, her eyelid is like a shut bud that holds a bee, her head droops like a fallen flower, and it is smiling and "rosy", which seems to accentuate her total subjection by him.