"After killing his arch enemy Moriarty Holmes becomes bored of life in England and decides to go to Rapture not because he believed in Ryan's radical ideals but because he thought it would help him avoid boredom."
"As Rapture marched on Holmes solved many criminal cases in Rapture, some of which involved influential figures in the city, such as Andrew Ryan and Frank Fontanie. The only thing that kept either from killing Holmes was Holmes blackmailing them into leaving them be."
"Having deduced Atlas' true identity Holmes could see the fall of Rapture coming from a mile away and so took the proper precautions, he annexed a part of Rapture for himself and filled it with deadly trap's keeping the other Splicers out of his domain and keeping himself safe."
"However like the other people of Rapture Holmes regularly used Plasmids leading to himself becoming a Splicer albeit a extremely intelligent one. The once great Sherlock Holmes having long gone insane now wonders the halls of his little fiefdom spends his days solving "crimes" of his own creation. Which are usually very graphic. Holmes is joined by "Watson" who is, in reality the long dead body of a man who's skull he caved in."
-Sorry for this being so long... i just had to get that idea out of my head.