The theme of good and evil in Stevenon's story relates to us today because there exists the eternal human dichotomy between good influences and evil influences, good impulses and evil impulses, good choices and acts and evil (or just bad) choices and acts. So whether today or the 19th century or post-2012, a story exploring the ultimate consequences of this dichotomy, and the dichotomy driven choices and actions, is always relevant.
The theme of good versus evil is explored in Dr. Jekyll and Mr. Hyde through the dual personas of Dr. Jekyll. Jekyll's friend, Dr. Lanyon, split ways with Jekyll long before the time of the story because he disapproved of Jekyll's "unscientific" experiments:
"But it is more than ten years since Henry Jekyll became too fanciful for me. He began to go wrong, wrong in mind; .... I see and I have seen devilish little of the man. Such unscientific balderdash, ... would have estranged Damon and Pythias."
Jekyll experiments with separating evil impulses from his psyche and personality so that he might fully indulge in those impulses without ill effect to his profession or reputation. Thus his good nature is left to continue to shine in the community unimpeded and unthreatened by secret vice. This separated evil persona took on a physical manifestation very different from Jekyll's own appearance. In this evil persona, Mr. Hyde trampled upon the fallen body of a girl whom he had collided with in a bystreet. Showing no remorse, only fear of discovery and punishment, he went on from there to commit more and greater acts of evil. His final act of evil being the brutal causeless murder of a high ranking dignitary for which he is hunted, because a witness gave a description, and for which he will be hung if caught.
Good is represented by Utterson whose psyche and personality are integrated with good and evil in correct proportion and working together for the benefit of all, including Utterson: his psyche is governed by moral principles, virtues and values. As evil grew stronger and more accustomed to public manifestation and exertion of will, good Dr. Jekyll became weaker and smaller and less able to face people or responsibility. In the end, evil overpowered good and subsumed Jekyll's weakness within Hyde's unrelenting evil strength.
Stevenson's message that evil, when given reign and not dampened by moral values and good impulses, will always overpower weakened goodness is as eternal as the dichotomy within between good and evil impulses.
And ... that insurgent horror was knit to him closer than a wife, closer than an eye; lay caged in his flesh, where he heard it mutter and felt it struggle to be born; and at every hour of weakness, and in the confidence of slumber, prevailed against him and deposed him out of life.