You have touched upon one of the main themes of the novel, the nature of good and evil. Mr. Hyde, the embodiment of evil and humanity's darker side, begins to take over Dr. Jekyll's life once he taps into that evil nature. Part of what makes us human is our struggle between our rational and good natures, personified by Dr. Jekyll, and our irrational and evil natures, personified by Mr. Hyde.
To refer to your essay question, this gets at the "nature vs. nurture" theory of human development. Are we predisposed to become what we are through 'nature' or our own hardwiring? Or are we a product of how we are 'nurtured' or the environment in which we grow up in? Your essay question falls in the latter category. Ask yourself how does Dr. Jekyll's life and experiences lead him down the path of corruption? Since he cannot come to grips with his more base urges (the darker side of his personality), he represses things and this leads him to develop the potion that he hopes will rid him of that side forever. Of course, it has the opposite effect.