The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson

Dr. Henry Jekyll, fascinated by the dichotomy of good and evil, no longer wants to inhibit his dark side.

He concocts a potion to create the alter ego of Mr. Edward Hyde. With the burden of evil placed on Hyde, Jekyll can now take pleasure in his immoral, nefarious fantasies-free of conscience and guilt.

It's when Hyde turns to murder that Jekyll realizes how monstrous his impulses are and how hard they are to suppress.