The poem's starts by setting the scene. It's a dark night, and the speaker finds a dead deer while driving down on a narrow road. The word "found" is important here, since it clarifies that the speaker wasn't the one to hit and kill the deer. Some other driver, it seems, struck the deer and didn't stick around to move it off the road. This carelessness, in turn, has created a dangerous situation for other drivers.
The speaker recognizes that the previous driver's decision to leave the deer on the road was irresponsible, saying, "It is usually best to roll them into the canyon." The word "usually" suggests that the speaker has encountered this situation before; it's apparently pretty common to find dead animals on the road that runs along the Wilson River.
This can be a very dangerous, the speaker indicates, since the road is "narrow" and the obstacle created by a dead deer's body could cause other drivers to "swerve" off the pavement—something that could "make more dead." In other words, the speaker worries that leaving the deer's body on the road might result in deadly car accidents.
The desire to avoid "mak[ing] more dead" suggests that the speaker feels responsible for other people's safety, wanting to decrease the amount of death in the world. This sets the stage for the rest of the poem, in which the speaker moves the deer in an attempt to reduce harm and loss—something that seems straightforward and easy at first but later becomes morally complex.
Alliteration and assonance evoke the imagery of these lines. The heavy /d/ sound in "deer / dead" places thudding emphasis on the phrase. The assonance of "dead on the edge" draws yet more attention to this image, making it hard for readers to overlook the image of a dead deer lying on the "edge" of a road.
Alliteration reappears at the end of line 4, when the speaker repeats the /m/ sound:
[...] to swerve might make more dead.
This alliteration again reflects the line's content: that flurry of /m/ sounds suggests the increase in death that will result if the speaker doesn't move the deer.