Robert Louis Stevenson explores the dual nature of man in this novel. He realized that man was created with warring natures - good and evil. Sometimes good overtakes man, sometimes evil. Man has a sin nature.
Dr. Jekyll was an upstanding scientist. He performed experiments for the good of society, but when left alone to his thoughts, he often obsessed over the fact that he also had an evil part of him that cared nothing for his fellow man. He did not like this evil side of himself especially, but it also was compelling to him. Both of these natures were truly him, but wouldn't it be nice if he could be happy when he was evil as well as be happy when he was good? If only he could separate the two!
“With every day, and from both sides of my intelligence… I thus drew steadily nearer to that truth, by whose partial discovery I have been doomed to such a dreadful shipwreck: that man is not truly one, but truly two… I saw that, of the two natures that contended in the field of my consciousness, even if I could rightly be said to be either, it was only because I was radically both.”
Ahaha! One night, he decides to concot a potion which will allow him to separate his two natures. That way, he could go from one nature to the other at will. At first, the good side is stronger. Whenever Jekyll got too caught up in his evil side, he could quickly drink the potion and get rid of Mr. Hyde, his alter ego. But before long, his good side grew weaker and he could no longer control his evil nature. Mr. Hyde began to transform Dr. Jekyll involuntarily until Mr. Hyde overpowered and finally destroyed the powerless Dr. Jekyll who by then was very sorry for what he had created, but alas, it was too late.
This is the great change of the story. The evil wins out because Dr. Jekyll's motives were not pure. He did it for ambition:
"Had I approached my discovery in a more noble spirit, had I risked the experiment while under the empire of generous or pious aspirations, all must have been otherwise, but at that time my virtue slumbered; my evil, kept awake by ambition, was alert and swift to seize the occasion.