The killer in A Study in Emerald is never revealed, but it can be inferred from the story that the culprit is none other than Edward Hyde. In this Sherlock Holmes-inspired short story by Neil Gaiman, characters refer to a mysterious figure as Hyde, who appears to have orchestrated a conspiracy involving Victor Frankenstein and Jack the Ripper. Furthermore, there are hints throughout the text of an ongoing rivalry between Mr. Hyde and Dr. Jekyll, which implies that Hyde has been involved with some kind of illegal activity for some time now. This leads us to believe that he had something to do with Professor Moriarty's death as well as being behind a series of murders in London during 1888--the same year Sherlock Holmes was active on Baker Street. The true identity of Mr. Hyde remains unknown at the end of the story; however, readers are left with enough evidence to draw their own conclusions about his involvement in these heinous crimes and ultimately assume he is responsible for them all.