A good thesis statement about good and evil from The Strange Case of Dr. Jekyll and Mr. Hyde is that good and evil are not distinct from each other but are intertwined and defined by each other. Dr. Jekyll, the quintessentially morally upright man in the eyes of society, has long contended with the secret darker side of himself. He writes a letter in which he explains his understanding of the dualistic nature of humans:

In his novel, The strange case of Dr Jekyll and Mr Hyde, Robert Louis Stevenson explores the duality of human nature - that man is, at once, both evil and good. In his characterization of Mr Hyde as Dr Jekyll's evil persona, he wishes to convey the intricate complexity faced by man: "What is my true nature, my true will? To exercise goodness and humility or to succumb to savage desire and degradation?"

Robert Louis Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde depicts a man torn between his "good" and "evil" self. This idea has been popularized in Gothic Romantic works (like Stevenson's text and Mary Shelley's Frankenstein). Gothic Romantic authors, when dealing with the doppelganger (a person's "twin"), play upon the idea that people possess both good and evil within themselves. While the amount of good and evil is unknown, everyone possesses the ability to be both good and evil. 

A thesis statement which addresses the idea of good and evil within the text is as follows.

The dual nature of human mentality, symbolized by the evil of Mr. Hyde and the good of Dr. Jekyll, illustrates the battle which rages within an individual. Robert Lewis Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde represents the battle between the intellectual and rational self and the irrational and animalistic self.