How might The Strange Case of Dr. Jekyll and Mr. Hyde be frightening to a Victorian audience?
In addition to sending the message to readers that they really want to be bad, this short novel also conveys the idea that our darker, more sinful desires are inherently stronger than our will to be virtuous. Dr. Jekyll tries to distill his urges to do evil, sinful things and separate that side of himself out so that it can be eradicated. The implicit message of such an attempt is that he, a person who seems to be a good, upstanding member of proper society, is not strong enough to resist those urges without assistance.
Second, the fact that the dark identity, Mr. Hyde, actually turns out to be more powerful than the side that longs to be virtuous confirms the message that our evil natures are stronger than our goodness. In the end, Jekyll loses control of Hyde, and the latter is able to emerge much more easily and readily than ever before. We see, then, that when they are pitted against one another, Hyde is more powerful, and, if he symbolizes the sordid, malicious side of human nature, his superiority says a lot about all of us. For the Victorians, who worked hard to repress "improper" urges and privileged virtue, the idea that it is ultimately a losing battle would be frightening indeed.
The Victorians were deeply concerned with character, with propriety, and with virtue. They were publicly committed to doing the right thing in a visible fashion; they wanted to be good, and to have their appearances match their character.
Jekyll and Hyde turned all those desires on their heads. It told them, "You don't want to be good. You really want to be bad—really bad!" It told them, "You can't trust someone based on appearances. Those change. You all have secrets."
It also addressed a fear of science. Science was engaged in overturning all that was precious to Victorians. There are lots of examples, but the most specific example is the theory of evolution that emerged during this time. Victorians wanted to be on the side of angels; evolution said, "You're essentially beasts!" So did Jekyll and Hyde.
To go along with the other posts, Dr. Jekyll essentially reduces the all-important, Victorian-revered soul to a science project. All he needed to do was find the proper ingredients and suddenly he had the key to controlling vice and virtue, the two qualities Victorians saw as being governed by the soul. Another element that would have clashed with a "respectable" Victorian audience was the blatant reference to Dr. Jekyll's taste for things not befitting his social status, and Hyde's complete abandon when prowling the streets. Before the book was even published, Robert Louis Stevenson was advised by his wife and others to edit the story so that it would not be so shocking to his readers, so the references to vice in the finished product the Victorians received was actually mild compared to what Stevenson initially wished to include.