The death in this poem is painless, yet the vision of death it presents is horrifying, even gruesome. The appearance of an ordinary, insignificant fly at the climax of a life at first merely startles and disconcerts us. But by the end of the poem, the fly has acquired dreadful meaning. Clearly, the central image is the fly. It makes a literal appearance in three of the four stanzas and is what the speaker experiences in dying.
The room is silent except for the fly. The poem describes a lull between "heaves," suggesting that upheaval preceded this moment and that more upheaval will follow. It is a moment of expectation, of waiting. There is "stillness in the air," and the watchers of her dying are silent. And still the only sound is the fly's buzzing. The speaker's tone is calm, even flat; her narrative is concise and factual.
The people witnessing the death have exhausted their grief (their eyes are "wrung dry" of tears). Her breathing indicates that "that last onset" or death is about to happen. "Last onset" is an oxymoron; "onset" means a beginning, and "last" means an end. For Christians, death is the beginning of eternal life. Death brings revelation, when God or the nature of eternity becomes known. This is why "the king / Be witnessed in his power." The king may be God, Christ, or death; think about which reading you prefer and why.
She is ready to die; she has cut her attachments to this world (given away "my keepsakes") and anticipates death and its revelation. Are the witnesses also waiting for a revelation through her death? Ironically the fly, not the hoped-for king of might and glory, appears. The crux of this poem lies in the way you interpret this discrepancy. Since the king is expected and the fly appears, are they to be associated? If the fly indicates the meaning of death, what is that meaning?