In Robert Louis Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde, one of the main themes in the story is the internal battle of human beings: the struggle...
...between good and evil in Jekyll's soul.
Is there a duality within a person? The story argues that such is the case. There is the perception of others to the two sides of a human being, as well as self-perception.
In viewing Hyde as an outsider, Enfield notes:
I never saw a man I so disliked, and yet I scarce know why ... He gives a strong feeling of deformity, although I couldn't specify the point ... I can't describe him.
However, Jekyll himself feels quite different at the start: it may perhaps be that he is too close to this other side of self to maintain an emotional distance...an objectivity—As Hyde...
I felt younger, lighter, happier in body; within I was conscious of a heady recklessness, a current of disordered sensual images running like a millrace in my fancy ... an unknown but not an innocent freedom of the soul. I knew myself at the first breath of this new life, to be more wicked, tenfold more wicked, sold a slave to my original evil...
Jekyll revels in being so different a person as Hyde—and it is intoxicating. While he knows he is in touch with a deep wickedness within, the freedom from the restrictions of society is invigorating.
Studying his alter ego, Jekyll alludes that the struggle to be a good person is an emotional and physical drain, while being evil enables him to feel less worn.
[T]he evil side of [his] nature…had been much less exercised and much less exhausted...
As time goes on, however, Jekyll is aware of the toll his evil and wicked other self has taken on the more civilized and proper Jekyll; it...
...had left on that body an imprint of deformity and decay.
Jekyll sees Hyde as monstrous. And Hyde—filled "with fear and hatred"—desires nothing more than to live on forever and see Jekyll simply disappear. It is a vision of the civilized man fighting his uncivilized, animalistic side—generally repressed in most humans.
Jekyll is something of a mystery to others; we can infer he has been that to himself as well:
Readers learn from Jekyll's confession at the end of the book that he was troubled by what he discovered in himself— "those provinces of good and ill which divide and compound man's dual nature."
While Jekyll's motives behind the experiment are "altruistic," like man stories revealing the struggle between good and evil with (such as the classic struggle within Darth Vader, for example), the story seems to note that one cannot dabble with the darkness within one's soul and be untouched by that darkness. In this story, Jekyll sacrifices himself in coming in contact with that side of himself that should have been repressed forever, allowing him to act in a civilized way. As in Shelley's Frankenstein, when Victor destroys himself with the custody of dangerous knowledge, this same element destroys Jekyll as well: for one cannot be two people. One is committed to being a good person or an evil person. Walking a line between the two shatters the natural balance of the civilized man. Jekyll cannot walk that line:
...this is my true hour of death, and what is to follow concerns another than myself. Here then...I bring the life of that unhappy Henry Jekyll to an end.