What causes a person to act in such radically opposite ways and what does this suggest about human personality or human nature?

Clearly, Dr. Jekyll and Mr. Hyde is an examination of the duality of human nature, as most clearly expressed in the revelation that Mr. Hyde is in fact Dr. Jekyll, only transformed into a personification of Jekyll's evil characteristics. Utterson's discovery of Jekyll's astounding work occurs in the final chapter of the novel, after Stevenson has laid the groundwork of evidence for the extreme duality inherent in human nature. We have already witnessed Hyde's powerfully vicious violence and have seen the contrasting kind, gentle, and honorable Dr. Jekyll. In approaching the novel's mystery, Utterson never imagines that Hyde and Jekyll are the same man, as he finds it impossible to reconcile their strikingly different behavior.

In pursuing his scientific experiments and validating his work, Jekyll claims, "man is not truly one, but truly two." Thus, in Jekyll's view, every soul contains elements of both good and evil, but one is always dominant. In Jekyll's case, his good side is dominant, but he knows there is evil inside of him. However, as a respectable member of society and an honorable Victorian gentleman, Jekyll cannot fulfill his evil desires. Thus, he works to develop a way to separate the two parts of his soul and free his evil characteristics. However, as Vladimir Nabokov explains in an introduction to the Signet Classic version of the book, "[Jekyll] is a composite being, a mixture of good and bad...[and] Jekyll is not really transformed into Hyde but projects a concentrate of pure evil that becomes Hyde." Unfortunately, rather than separating and equalizing these forces of good and evil, Jekyll's potion only allows his purely evil side to gain strength. Jekyll is in fact a combination of good and evil, but Hyde is only pure evil. Thus, there is never a way to strengthen or separate Jekyll's pure goodness. Without counterbalancing his evil identity, Jekyll allows Hyde to grow increasingly strong, and eventually take over entirely, perhaps entirely destroying all the pure goodness Jekyll ever had.

Other theorists have argued that perhaps Stevenson concludes that man is not in fact a purely dual being, but is at his heart a primitive being, tamed and civilized by the laws of society. Stevenson does portray Hyde in highly animalistic terms - short, hairy, and like a troglodyte with gnarled hands and a horrific face. In contrast, Jekyll is described in the most gentlemanly terms - tall, refined, polite and honorable, with long elegant fingers and a handsome appearance. Thus, perhaps Jekyll's experiment reduces his being to its most basic form, in which evil runs freely without considering the constraints of society and civilization.