A central theme explored in the book is the conflict between good and evil, highlighting how perceptions of these concepts can be misleading. For instance, Shadrack, introduced in the first chapter, is viewed by the townspeople as dangerous and malevolent. When he says "Always" to Sula, she interprets it as a threat. In reality, Shadrack is not evil; he is merely traumatized and misunderstood. Throughout the narrative, he never harms anyone. Similarly, Sula is labeled as evil, particularly in the latter half of the book, while Nel is seen as good. However, by the story's conclusion, Nel comes to realize she harbors malevolent thoughts and has committed wrongful acts, whereas Sula has inspired some of the most virtuous deeds the town has ever witnessed.

Sula's grandmother, Eva, is regarded as virtuous, respectable, and a community pillar, yet she possesses a darker side. Her ruthless nature is suggested by rumors that she orchestrated the amputation of her own leg. This act is mirrored by Sula, who cuts off the tip of her own finger to intimidate some harassing white boys. She tells them that if she can do that to herself, they should consider what she could do to them. Sula's small act of self-harm is insignificant compared to Eva's, raising the book's implicit question: "If she's willing to harm herself, what might she be capable of doing to others?" The answer is, "Anything and everything," including murdering her own son by dousing him with kerosene and setting him ablaze while he is in a drug-induced stupor.