What was the monkey in the Chinaberry tree catching?
When the family arrives at Red Sammy's barbecue restaurant in "A Good Man Is Hard to Find" the children spot a monkey chained to a tree. When the children approach, the monkey runs away.
The monkey is frightened of the children and recognizes them as a threat (even though the monkey has never seen these children before). The monkey acts on its survival instincts.
Instinct comes in to play later in the story when the father character crashes the car. The reason for the crash stems from the man's instinctual response when the family cat jumps on his neck. He loses control of the car and crashes it.
The family is too shocked to act on instinct minutes later when the Misfit pulls over to "help" them. Like the monkey, they should have fled or hidden or done something to escape, but the family did none of these things. Instead, they passively agree to walk into the woods where they are shot one by one.