The snake is the most powerful symbol in the novel, included even in the title. The Serpent King alludes to the three generations of the Early family, all three men (grandfather, father, and son) named Dillard to emphasize the line that runs between them. The author connects the snake differently to every character: For the grandfather, the snake symbolizes madness, and the insidious nature of his profound grief; for Dill Sr., betrayal, darkness, and danger; for Dill, the fear of becoming like his father and grandfather. 

The snake as a biblical reference is important in this novel because of the religious belief and practice in the Bible Belt. In the Bible, the betrayer of Paradise is a snake, who offers Eve the apple from the tree of knowledge, thus leading to the original sin. The snake also symbolizes temptation: Each time Dill visits his father in prison, Dill Sr., who has succumbed to temptation, has more snake tattoos coiling around his arms. For the same reason, the author portrays Dill Jr. as being afraid of snakes, just as he is afraid of making a stand against his family legacy until he matures enough to know better.