The role of mad scientists have a rich tradition in cinema, defining an entire genre unto itself. It is but only a variant on the " villain" character, to help set up the classic backdrop of good vs. evil. (They sometimes also tend to be a close cousin to the "absent-minded professor" type, but these are usually only bit characters, not applicable here.) Everyone knows that a story with only good people with happy lives makes for dull storytelling.
Mad scientists and inventors occupy a particularly celebrated role. Why is this? Most people hold science and scientists in high regard. Is this caricature a reflection of what they really think of them? Like other classic characters, do we truly believe them to be as common as the crooked politician, the overbearing boss, or the creepy guy in the trenchcoat?
Looking at the most often repeated profile, the scientist or inventor in question typically starts out good- or at least relatively normal. At one point, however, there is usually the advent of some monumentally coveted "thing"- world peace, immortality, a cure for juvenile delinquency. Being a man of science, (as opposed to evil), there is usually an element of selflessness....this "thing" appears as universally beneficial for the greater good- or at least you can't blame him for wanting it and believing in it. Outwardly, we the audience empathize and embrace the "knee jerk" reaction. It is for science, it is for the betterment of society, the inventor of the thing deserves a medal.
Of course, it is not so simple. The author of the tale begins to poke holes in this simplistic theory, and the principle begins to unravel. Often, the viewer notices and uncanny parallel with real issues, with the historical or current backdrop, forcing us to question our own real-life dilemmas, casting doubt on our core beliefs....sometimes it is not so pleasant.
This is classic storytelling. The minute we are reminded of these real life parallels, the lines are blurred, and we begin to feel empathy- we feel we have a stake in the outcome.
As such stories pivot around a dilemma, there is often one character that does not seem to exhibit this doubt. The scientist is so close to the action that he does not see the larger picture; he is singleminded in his quest to give closure to the original plan.....Damn the consequences!
It is not he who has turned from good to evil, it is we the audience who has turned our perception around.