Both novels deal with the scientists upsetting the laws of the universe. Both of the scientists are dabbling in the unknown, attempting to play God. In the case of Dr. Jekyll, he's trying to understand the dual nature of humans to be both good and evil, and he wants to separate the two. Dr. Frankenstein tries to balance his intellectual and social interactions.
Dr. Jekyll thinks if he can rid man of his evil side, man will be better off because "life would be rid of all that was unbearable; the unjust might go his way delivered from the aspirations and remorse of his more upright twin; and the just could walk steadfastly and securely on his upward path, . . . no longer exposed to disgrace and penitence by the hands of his extraneous evil." By separating the two, Jekyll would be separating the moral and the intellectual, and he felt man would be better off for it. Jekyll became too ambitious however.
Dr. Frankenstein's creation has also been referred to as his dark side, the embodiment of Frankenstein's evil self. Victor, like Jekyll, is science gone amok. He refuses to take responsibility for his creation, and innocent people are hurt, just as they are by Mr. Hyde. Both doctors don't understand human nature, and as a result, their results are disastrous. Both are also overambitious, allowing their experiments to control them. They lose sight of their goal, and in the end, they allow their evil sides to triumph.