Why did Robert Louis Stevenson write "the strange case of dr jekyll and mr hyde?

Stevenson had long been interested in the idea of the duality of human nature and how to incorporate the interplay of good and evil into a story. While still a teenager, he developed a script for a play on Deacon Brodie, which he later reworked with the help of W. E. Henley and saw produced for the first time in 1882.[3] In late 1884 he wrote the short story "Markheim," which he revised in 1885 for publication in a Christmas annual. One night in late September or early October of 1885, possibly while he was still revising "Markheim," Stevenson had a dream, and on wakening had the intuition for two or three scenes that would appear in the story. "In the small hours of one morning," says Mrs Stevenson, "I was awakened by cries of horror from Louis. Thinking he had a nightmare, I woke him. He said angrily, 'Why did you wake me? I was dreaming a fine bogey tale.' I had awakened him at the first transformation scene ..."