From Dr. Jekyll's own account, he has had in him prurient desires that he secretly has satisfied. When he tries to separate them from his professional identity by transforming his appearance so he can conduct his immoral activities without recognition, Jekyll inadvertently has strengthened these desires by providing them essence in the person of Mr. Hyde. Indeed, this creation of a being in whom these immoral urges can be separated from the person of Dr. Jekyll gives them strength as they have their own identity.
By Chapter 7, then, the evil side begins to dominate Jekyll as Mr. Utterson and Mr. Enfield take their Sunday walk. For, as they speak to Dr. Jekyll beneath his window, they notice a remarkable change in the features of the physician.
...Mr. Utterson at last turned and looked at his companion. They were both pale; and there was an answering horror in their eyes.
In Chapter 8, Jekyll's manservant Poole seeks the help of the lawyer Mr. Utterson, whom he leads to the cabinet door of Jekyll's laboratory. The man within, Poole contends, is not his master. He is shorter, more of a "dwarf' and his face appears masked. It takes more and more of the potion for Jekyll to return himself to normal. For, Hyde is pure evil, but Jekyll is both evil and good mixed together. Thus, since there is less in Jekyll that is good than what is evil, it becomes easy for evil to supersede good.