The imagery here is interesting. Beginning the poem with imagery using many verbs and non-noun words such as back, shot (twice, emphasizing "He came back"; as if to say the killer returning to shoot was like twice intentional), fell, stumbling, down, shot (again), dying, dead. Perhaps the lack of nouns and metaphor project a more literal feeling.
Only in the second paragraph do we get introduced to the nouns: speeding bullet, tearing his face, "blood sprayed fine over the killer and the grey light". Now we have the characters missing in the first stanza, and "the grey light"; perhaps even a pseudo-metaphor. As seen in the third stanza, the light is only a metaphor, the darkness both literal and metaphorical, as is the tumbling.
The journey from literal to abstract is gradual: the first metaphor of light so commonly (over)used and familiar, it can almost "feel" real to the reader. This sets up the metaphor of tumbling down the stairs, more thoughtful and insightful.
The unusual break in the middle signifies an end and a beginning in middle of the poem; perhaps because we return to the literal.

