The black snake in Mary Oliver's poem must be considered, on the simplest level, as a character in a story. The snake slides onto the road, "the truck could not swerve," the snake is killed, and the narrator carries the dead snake into the bushes.
On a deeper level, this character, and the story it is involved in, serve as a vehicle for the poet to consider the themes of death and life.
After disposing of the snake, the narrator considers some of the negative aspects of death:
its terrible weight,
its certain coming.
Death, however, does not extinguish the glory of life. There is a "brighter fire" that "says to oblivion: not me!"
There is a "light at the center of every cell," a mysterious life-force, whose beauty--even if temporary--is so great that death cannot darken it.