"The cause of the explosion?" Sherlock asked Lestrade later, still sitting in the waiting room. He couldn't go home, not yet. "Was it a bombing? Targeted? Chance?" he spat the last once out like it was toxic.
Lestrade winced. "It was a meth lab that exploded. The two guys who were in the building were dead when the fire crew found them."
Chance.
John was only dead because he happened to be in Hoxton at that time, happened to be passing by that building at that time, happened to injured in such a way that it started a bleed in his brain, but also caused internal bleeding, happened to show no symptoms of the brain bleed, happened to be fine when he went into surgery, and brain dead when he came out.
All those tiny things, and without one of them, none of this train of dominoes would have come tumbling down.
But wasn't it like that for everything?
Because he'd only met John because he happened to see Mike that day, happened to tell him he was looking for a flatmate, because Mike happened to be sitting there when John walked by, and John happened to be walking by, because John happened to agree to coffee and mention that he couldn't afford London and Mike happened to remember what Sherlock had told him, because he happened to be at the hospital when Mike brought John by to introduce him, that John happened to be mad enough to come, and even madder to stay. All those things were chance too.
And Sherlock supposed, if one of those tiny things had been changed, John wouldn't be dead now.
Of course, he could have been dead a long time ago. Perhaps John was walking home that day to get his gun and shoot himself.
Sherlock shuddered to think about it.
But still, he wanted to be able to trace all of those threads back to the beginning, untangle them, and see when they could have gone instead.