Bored.
No drugs today. It was an experiment. He'd go until he either got a case or went literally mad, just to see how long it would take.
He tried reading. He tried wandering the Internet. He tried, just for fun, changing the password on John's laptop to see how long it would take him to figure it out. (John surrendered after fifteen minutes)
The second day was a near-repeat of the first.
The same for the third, fourth, and fifth days.
The sixth day, he fought the drug cravings, insidious, creeping. He had to fight that reminder of the old way out. It was not easy.
By the start of the second week, he'd tried sleeping through the boring hours, forcing himself to skip time the best way he could think of that didn't involve drugging himself.
Day ten, the insomnia hit. In order to fight it, he gave himself puzzles. Crimes, plotted in his head. First he'd play criminal, then he'd play police. Mental chess against himself. He started with a robbery.
Don't speak. Touch nothing. Could still identify via handwriting. Collage-written notes. Source magazines traceable. Simple pointing of firearm is enough to communicate intentions. Gait, model of weapon, height, all identifiable. (Mask a given.) Wear clothing atypical of own wardrobe. Style hair differently. Most people are uncomfortable in foreign clothing styles—easy to tell if in disguise. Not me.
It was about that time that he noticed he'd lost sensory awareness, and mentally smiled.
It became a habit with him, thinking up more and more complex crimes as he fell asleep, staving off the boredom. More than that, an addiction. From bank robberies to assassinations to grizzly mass murders, the more complicated the better. The more detailed the crime, the more he lost himself in details, the sooner his senses shut off.
But soon, mental planning wasn't enough. Before long, he had dioramas of his crimes spread across 221B, getting strange looks but no questions from John (who presumably knew better). After that, John acted them out with him, uncomfortable, but not refusing.
Then one night, Sherlock didn't return home until three in the morning, grinning.
"It worked, John!"
"…what?"
"Oh, that's brilliant! I always thought that solving crimes was the more fascinating aspect, that knowing both the start point and the end goal made it less interesting, but working within the constructs of those goals is much more interesting than anticipated!"
John stared at his elated flatmate. "What did you do?"
Sherlock tossed a bag at John. Full of cash.
"Did you…did you rob a bank?"
"I'm not making you an accessory. You're a rotten liar."
John cleared his throat. "Uh…okay." But it wasn't. Donovan had been right. He'd committed a crime out of boredom. Thank God it hadn't been murder. Sherlock still smiled as he went into his room and crawled into his bed, falling asleep peacefully for the best night's sleep in weeks.
The next day, John made certain that Sherlock returned the money and gave him a stern reprimand. The detective didn't seem to listen. This was not good.
"We've got something for you," Lestrade said, standing over the body of a woman whose head was in a plastic bag. "Kathy Richardson, apparent suicide, but—"
"But you have reason to think otherwise."
"Exactly." Lestrade couldn't place it, but there was something off. Every bit of fact screamed suicide, but something in his gut told him that there was foul play involved. And something in Sherlock's look was off, too.
"You're wrong. Obvious suicide. Listen to the facts, Lestrade, don't waste my time."
John's stomach twisted. He knew that tone. It was the tone Sherlock used when he was denying his drug use. The tone he used when trying to convince John that there was nothing wrong when there very clearly was something to be worried about. The tone he used when he was guilty and hiding it.
Sherlock had killed that woman.
Because he was bored.