Robert Louis Stevenson 1886
Great story! Make sure to add this short novella to your list of classics to read. It has suspense, angst, mystery and admirable development of empathy for the main character for such a short text. The setting offsets the plot and amplifies it flowingly.
A very good man ventures into the land of the alter ego and the impulses not followed by decent people. With a formula he discovers, the doctor unleashes the pure essence of his evil side. His fascination with the other side is his downfall, because an impurity in the powders cannot be replicated, and he will be trapped forever, his own personality repressed while the beast rages on. So in a last act of unselfish goodness, he manages to end his nightmare and spare society an evil madman.