Every decision leads to a new universe. Every time you turn left instead of right, get coffee instead of tea, you make an entirely new universe, all of which branch out from you like roots on a plant, always stretching for more.
And in every universe, there is a version of you.
Or at least it started out that way. Because all those different decisions, those tiny little choices that differ from others, could mean the difference between life and death.
So in some universes, you are not alive, having stopped that extra minute to smell the flowers, only to be run down by a bus that didn't see you. In other universes you are almost the same, except for perhaps a different colour hair, or a different favourite food. But no two universes are exactly the same, no two versions of you identical. Because the smallest decisions, they can make the most massive difference.
In this universe, Sherlock Holmes did not become a consulting detective, but a criminal.
And as far as criminals go, he is a damn good one. He enjoys the game as much as he does the crime, leaving trails of breadcrumbs that the Yard more often crumbles under their feet than they do follow them. Sherlock does not kill anyone in his time as a criminal, preferring to pull clever heists and blackmailing gigs than to get his hands dirty with something as lowly as murder. He could, if he wanted to, of course, but it never comes to that, and he retires early, moving out to Sussex to keep bees.
In that universe John Watson gets shot in Afghanistan and returns home to London, meeting Mike Stamford that day, but after sharing that he can't afford London on an army pension, Mike has nothing to offer, and John is forced to move in with Harry. He hears of Sherlock's doings, something about a painting, and another time with a large scandal involving someone high up in the British government, but never knows of the man behind them, the man who he is best friends with in another universe, a man who saved him. John keeps his cane until he dies in that universe.
In a different universe, Sherlock drowns instead of Carl Powers. Moriarty finds no pleasure in his first murder, and does not go on to become a master criminal with his services for hire. He works in IT at a local hospital, and meets a girl there who is shy and lovely. He moves in with her and her cat; she introduces him to Glee. John does not go on to study medicine in that one, but instead becomes a teacher. He is endlessly patient with his students, and he is well loved by them. He marries one of his fellow colleagues, and they grow old together. John dies in his eighties, never having know what he missed. But if he was ever asked, would be appalled at the sort of life that was spoken of.
In one universe, Sherlock is never born. John never knows about the missing part of him, but goes through his life feeling empty, no matter how many lives he saves, no matter how many bullet wounds he sutures or how many hearts he wills to continue beating.
He can't force his own to, and dies at the age of 56.