It was a danger night, that much John had sensed since he arrived home from the clinic.  As he turned the knob to enter 221B, he found the flat in shambles. The coffee table, which had this morning been covered in notes, files, and photos related to the case they were working on, looked as if it had been swept clean. All their research and the shattered remains of a teacup lay on the floor. The tea had almost certainly stained the carpet, but John didn't care.

Instead, he was worried about Sherlock.

He first exited the apartment and wrenched up the step third from the top. The drugs and cigarettes were there.

Sherlock hadn't found them yet, at least. John let out a breath as he reinstated the step and reentered the flat.

Not knowing what else to do, John made a very strong cup of coffee. One of his friends from the army, a reformed addict, had told him that sometimes a really dark roast might just keep you from falling off the wagon, if only because holding it gave you something to do with your hands.

"Sherlock?" John asked, nudging the door to Sherlock's bedroom open with his foot, hands full of hot beverage.

Sherlock was sitting cross legged on his bed, sifting through a shoe box, pulling out picture after picture, paper after paper. 

John watched silently for a few moments before he said again, "Sherlock?"

It was the one and only time he ever startled the detective. Sherlock jumped, head snapping up. Recovering from the initial shock, Sherlock stuffed all of the mementos lying scattered around him on the duvet back into the shoe box and slammed its lid. 

"What the hell, John?" Sherlock shouted, hastily jamming the shoe box under the bed. The sleeve of his shirt was rolled up, and John noted with growing anxiety that the other man had four nicotine patches on his arm. "You can't just bloody well barge in like that whenever you please."

"I knocked," John said absently. Sherlock's eyes were red. Not drug red, almost...

Crying red?

Sherlock inhaled deeply. "Right. Right. I'm sorry, I just- John, just give me some space, will you? Just for a few hours."

John nodded, dumbfounded. Sherlock apologizing? This was even bigger than he'd originally thought. He had just reached the door when he heard from behind him-

"Leave the coffee."

                                                                                                ***************

"Leave the coffee."

John shut the door behind him, and Sherlock collected the drink before reaching to pull out the shoe box again.

He hesitated. Should he? Was it worth it?

Of course it's worth it. You're dying, for God's sake.

Sherlock pulled out the box, removing the most recent addition to his collection, Molly's newest report. 

It had spread to his liver. Molly said there was nothing she could do. He had a month, two if he was lucky. 

Sherlock cast a glance towards the door his boyfriend had just exited.

It would hurt him so much. John would miss him, wouldn't he?

And in that moment, Sherlock made up his mind to remedy this.

                                                                                                  *********************

John awoke late the next morning to a silent flat, which was unusual. Sherlock usually woke John up whenever he got bored of being by himself. He poked his head into Sherlock's room, which stood empty. A quick check revealed that the drugs were still under the step. He was on a case, most likely.

John headed for the kitchen to make himself breakfast, and as he went to open the fridge, noticed a piece of paper taped to the door, written in Sherlock's unmistakable narrow, jagged writing.

John,

     I'm afraid I've made a mistake. Forgive me, I was...confused following Irene's death. I did not love you. Well, I did, but only in the most platonic of senses. 

Something was scribbled out.

Another scribble.

You were a mistake. I'm leaving. I can't bear to go back to the way we were before, as if nothing happened, when something so irrevocably did. 

I'm so sorry, John. Don't come looking for me, please.

-SH

Dr. Watson cried that day.

And then he became so, so angry.

                                                                                          *****************

Sherlock Holmes, content that his ex would not mourn for him, died three weeks later in an unmarked grave, the way he had asked his brother.

Mycroft, Molly, and Greg were the only attendees of his funeral.

                                                                                          ********************

John Watson, mourning for his ex, died seven weeks later, by his own hand. He was buried, in the sort of cosmic mockery we call a coincidence, just a few plots away from Sherlock.

AN: Got a favorite Sherlock headcanon? Post it in the comments and I'll do my next chapter on it. The idea well runs dry.