Dr. Jekyll and Mr. Hyde (1931)

"Put yourself in her place! The dreaded night when her lover became a madman!"

Dr. Henry Jekyll believes that there are two distinct sides to men: one that is good and one that is evil. He believes that by separating the two, man can become liberated. He succeeds in his experiments with chemicals to accomplish this and transforms into Hyde to commit horrendous crimes. When he discontinues use of the drug, it is already too late.