Symbolism, Imagery, Allegory
In literature, forests get a bad rap.
Little Red Riding Hood gets tricked by a wolf in a forest. Dante starts his descent into hell wandering around in a forest. The Forbidden Forest near Hogwarts gives Harry Potter almost nothing but trouble. Forests are full of twisted branches and creepy fungi and deceptive little critters and jagged rocks. To read almost any book (and certainly any fairy tale) is to understand how purely evil forests are. In fact, when you start reading, it seems like only good thing to have come out of forests is the Black Forest Cake.
And then, when you read "A Good Man Is Hard To Find" you know that the only good thing to come out of forests is the Black Forest Cake.
Because the forest that looms over the ditch where the family's trapped after their car accident—"tall and dark and deep" (70)—is a threatening image that suggests the impending death of the family.
And, what do you know: the forest is where everyone except the grandmother is taken to be shot. Literary forests suck.