Disclaimer: I do not own Sherlock, Watson, or anything besides the plot. They all belong to BBC, if I did I would have a lot more money and would have Benedict Cumberbach here, not just be drooling over him online with everyone else.
Enjoy.
He couldn't take it anymore. Sherlock couldn't be gone, he just couldn't.
For the first couple of months he kept going on like normal. Bringing in the paper each day to his Sherlock, designed from a mop and a cap with a gun attached with a string he could pull to make it go off when things got to quiet. He would read off the new murders, kidnappings, rapes and hope that maybe one day a crime would come up big enough that Sherlock would have to come back just to solve it. Watson knew his old friend well, knew he could never miss a challenge. But London remained quiet, and Sherlock remained absent. That absence ate away at him, the pain in his leg returned worse than ever, the pain from all the things he had never said coming to bear on his limbs.
And then something just snapped. And then he just could not wait any longer. If no one was going to step up and create a crime heinous enough to bring Sherlock back, well damn it he would do it himself.
Watson had never been a cold blooded killer, unable to kill anyone who was innocent he started murders in homage to his friend, his love, while hoping he would come back to investigate. To catch him, it all was easier than he thought. He had learned investigating skills from Sherlock, though he would never presume to be as good as Sherlock's genius, and he finally had a way to put it to good use. He started small, killing a murderer here and there as quickly as possible. Laying the bodies of the men and women who Sherlock should have been catching on his grave as though the proximity to the mystery would bring him back sooner.
But it didn't, it wasn't enough, and Watson began to get impatient. And he began to get sloppy. His victims, now with lesser and lesser crimes, were tortured for days in Baker Street 221 while Mrs. Hudson fled the city, fearing she might end up next on the executioner's block if she stayed, remarking to herself as she left that Watson had always seemed like such a nice, quiet man and blaming Sherlock for ruining him.
The stack of bodies on the grave grew larger every night as he grew more and more impatient; screaming at his fake Sherlock as though the mop wearing his friend's hat could respond. "Sherlock, Sherlock, there has been another homicide!" But there was never a response. No one ever came to catch him. Even the screams of his victims would not summon Sherlock from the grave. It was Lestrade who finally found him, kneeling on the grave with bloody hands and a body in front of him.
"Watson, please, tell me you didn't do this."
"He didn't come. I thought for sure he would come, he always used to come."