What is the main idea of the poem?

The main idea of the poem "Traveling Through the Dark" by William E. Stafford is subtle. In the poem, a driver finds a deer in the road that was recently hit by a car. He thinks about rolling the deer out of the way and into the canyon in order to prevent future drivers from swerving around the body, which "might make more dead." As he drags the deer to the edge, he notices that she had been pregnant with a fawn who is seemingly still alive:

her side was warm; her fawn lay there waiting,alive, still, never to be born.

The realization makes the speaker hesitate, and the poem goes silent and still for a stanza. The speaker notes the glow of his headlights, the "purr" of his engine, the red-tinged exhaust coming from his car, the wilderness still and "listen[ing]." He finally decides, after this extended moment of thought, to push the doe over the edge of the canyon.
The poem ultimately addresses questions of what we owe to other animals. In showing the simultaneous mercy and what some may see as insensitivity in the speaker's choice to roll the doe into the river—an action that both kills the unborn fawn (who would have died, in any case, without its mother) and saves future drivers and animals from harm—Stafford draws a thin line between the tenderness and cruelty that underline all our animal lives.
This is true for deer as well as humans, here, as Stafford emphasizes "our group" (made by the driver, the doe, and her unborn fawn) in the first-person plural. In this, the poem sits on the knife-edge between the past and the future: our actions and decisions in the present, our "swervings," have material and potentially fatal impacts for both human and nonhuman animals. We have life and death in common.
https://www.poetryfoundation.org/poems/42775/traveling-through-the-dark