Noel Carroll, in The Philosophy of Horror, explains that in over-reacher plots, the experiment performed by the protagonist often goes awry. Does Dr. Jekyll's experiment fail? Does the potion do what Jekyll hopes?

I think that Jekyll gets more than he bargained for. Although his experiment worked, it works too well. There is always a Faustian bargain to these types of scientist gothic-like stories (ie Shelley's Frankenstein). Jekyll gets what he wants, the unfettered freedom of depravity, but it comes at a terrible cost.