The speaker of the poem is reacting to their lover the way that one would react to a decaying corpse as they compare the ending of a relationship to death. The speaker says that the time for pity is in the moment of death, implying that they felt badly for their lover only in the moment that the relationship was ending. Now that the discarded lover has had time to process these feelings and yet is still "self pitying," they appear before the speaker as a decaying corpse, inspiring only revulsion.

The poem is not all filled with hatred, however; the speaker tells their jilted lover to come back when their "bones are clean," meaning that once the discarded lover has abandoned all the toxic behaviors of someone wallowing in negative emotions, the two can form a new sort of connection.

The central metaphor of the poem is a comparison between a dead then decaying corpse, and a "discarded" lover.  The lover is the tenor of the metaphor, and the corpse the vehicle by which the speaker reveals a more complex meaning for the lover. 

The speaker uses grotesque images of decay to demonstrate what the discarded lover is like for him in the poem's present:  dead bird, creeping stench, wriggling, munching scavengers, maggots close to the surface. 

In time, the lover will become as a skeleton, rather than a rotting corpse:  a shape of clean bone, an inoffensive symbol.

The speaker plays on the difference between a fresh corpse--the kind one calls the police about if it's human--and the kind you call an archaeologist about; one is repulsive, the other is scientifically interesting.  One is offensive, one is not. 

The second-person speaker speaks directly to the discarded lover in a harsh, sardonic tone.  The speaker is unforgiving, though he/she does suggest the detachment that time produces will make his view of the lover like a clean skeleton, rather than like a fresh, rotting corpse.