A speaker comes across a corpse in the street at night, and the poem moves in a few quick, broken steps. The first lines set the scene plainly: darkness, a body, nothing else. Then comes fright, and with it a mention of an "oath." The poem never says what the oath is, so we have to guess. It might be a promise to help, a vow of some kind, or a pledge the speaker feels they are failing to keep in the face of death. That gap is part of what makes the poem unsettling.
The last line, "a wing," arrives alone, almost like a flash seen out of the corner of the eye. It could be a bird startled in the dark, or an angel, or a sign of the spirit leaving. The poem doesn't decide. The mood is hushed and tense, caught between terror and something close to awe, and the ending leaves us suspended there.