Dr. Jekyll is afraid of Mr. Hyde because he doesn't know what he might do in that other personality. The story was written in Victorian times when much that is commonplace in modern fiction was absolutely unmentionable then. Mr. Hyde is represented as a totally fiendish, ruthless, sadistic man, but most of what he actually does on his outings is only implied. Buy Dr. Jekyll knows that if he were to lose complete control of his civilized persona, he could commit an endless series of horrible crimes against men, women, and children. He also realizes that if he went on a crime spree in London, it would only be a matter of time before he was arrested and exposed.
The author of "Dr. Jekyll and Mr. Hyde," Robert Louis Stevenson (1850-1894) shows the influence of two American writers, Nathaniel Hawthorne (1804-1864) and Edgar Allan Poe (1809-1849). Hawthorne dramatized the existence of evil in man in stories such as “Young Goodman Brown,” “The Minister’s Black Veil,” and “My Kinsman, Major Molineux,” as well as in his novels, most notably The Scarlet Letter and The House of the Seven Gables. Poe revealed the evil side of man in “The Cask of Amontillado,” “The Tell-Tale Heart,” “The Black Cat,” and other short stories. In “The Strange Case of Dr. Jekyll and Mr. Hyde,” Stevenson seems to be acknowledging the existence of an evil side of human nature but speculating on the possibility of eradicating it. Stevenson is more like Hawthorne than like Poe. Hawthorne usually illustrated the fact that most people try to keep their evil side hidden, whereas Poe wrote in such a way that the good side, if any, was what was hidden. Hawthorne tended to write in the third person, making it much easier to present his characters from the exterior, from the appearance they made in public. Poe tended to write in the first person, putting the reader right inside the mind of the character who was favoring the evil side of his nature.
There is a lot more evil in human nature than Stevenson, Hawthorne, or Poe ever dared to unveil, but many sins, crimes, perversions, and sadistic actions simply could not be mentioned in those straitlaced times.
When Dr Jekyll becomes Hyde he turnes into a shorter, hairier young man who looks nothing like himself. This is one of the reasons that Jekyll is so attrated to his alter ego, it gives him the ability to whatever he pleases and not have to suffer the conseqences that may affect his status in society. Jekyll is scared when he no longer has control over Hyde.