I don't want to talk about it too much because I'm more conscious than ever of spoilers, but needless to say I didn't guess who the killer was at all, although much of the time I don't think the viewer is really supposed to.
Back in the day, the BBC Cult website housed a Sherlock Holmes fiction page which includes a short story featuring a missing adventure for the Conan Doyle original, The Deer Stalker and it's still online. Says Paul:
""I've always been fascinated with the gaps in the Sherlock Holmes stories, the stuff that we don't see about how Holmes functions as a real person. Conan Doyle, I think, realised that Holmes is one particularly attractive/ugly aspect of the male psyche made flesh, almost incapable of a complete literary life, and so suggested that Holmes 'switches off' between cases with his drugs, almost as if he really doesn't exist when one isn't reading about him."You can read the story here.