He was a most peculiar man

That's what Mrs. Hudson said, and she should know
She lived downstairs from him
She said he was a most peculiar man

Mrs. Hudson watched as Sherlock came through the door, he bounded up the stairs without a word. Ever since his narrow escape from the cab incident, he'd become even more reclusive than before. It was a good thing that the murderer's aneurysm had chosen that moment to burst or the world may have been lacking its only consulting detective. Mrs. Hudson bit her lip and continued to make her tea. Sherlock didn't really talk, so it probably wasn't her place to make conversation with him.

He was a most peculiar man
He lived all alone
Within a room within a house
Within himself

Sherlock dropped his coat to the ground, his stuff was littered throughout the flat. There wasn't anyone there to get after him and Mrs. Hudson had long since given up on trying to keep Sherlock's flat clean. In fact she barely set foot in it anymore. She used to occasionally chide to keep it clean for visitors, but that had stopped as soon as she came to the realization that no one ever called on Sherlock. In fact, the only time there was any coming and going from the flat was Sherlock going to and returning from cases which were becoming increasingly less often.

A most peculiar man
He had no friends, he seldom spoke
And no one in turn ever spoke to him
Because he wasn't friendly and he didn't care
And he wasn't like them
Oh no, he was a most peculiar man

Sherlock bent over his microscope, engrossed in his experiment. It was now late at night, his case from early that morning long since gone. Sherlock checked his phone again, maybe Lestrade had- no. No texts, no calls. Perhaps Sherlock shouldn't have detailed exactly why the Detective Inspector was incompetent, but Sherlock had solved the case. Oh well, he didn't care about Lestrade's or Donovan's feelings. Especially not Anderson's. He just wanted a case to keep himself occupied, why would they expect him to conform to arbitrary social norms just so that he could solve their case? It didn't make sense. They knew he didn't care or work like that, so why hinder him with it? He didn't understand their simple little minds. He would bet that it was peaceful in there… No such luxury was afforded for him, he had to suffer his brain's constant striving, thinking, and reaching. It got quite exhausting sometimes.

He died last Saturday
He turned on the gas and he went sleep
With his windows closed, so he'd never wake up
To his silent world in his tiny room

It was only the logical decision. Sherlock was in pain which he found intolerable. Drugs couldn't help it, cases were making it worse and he found himself incapable of coping with it any longer. Unlike how Mycroft would have predicted, it was a quiet, undramatic affair. Sherlock spent the morning compiling his legal documents and jotting down a brief explanation to the police. It would be no good for that idiot Lestrade to go and think his Consulting Detective was murdered and send valuable resources on a wild goose chase for a criminal that didn't exist. He dressed himself in his favorite purple shirt, his best pair of dress pants and the jacket he loved so dearly. He startled himself as he noticed his hands were shaking as he turned on the valve for gas in the apartment. He started at the offending appendages with mild surprise. Apparently he was more nervous than he thought. Despite his shaking hands, he continued to turn the valve that would end his life.
He made his way to the couch where he enjoyed sleeping the most and curled up one final time. As his eyes began to close, tears trickled out of them and down his cheeks. They stained his fair face as they dripped onto his pillow. No longer bewildered at his own emotional reactions to his own death and pain, Sherlock drifted into a cold sleep from which he would never wake.

And Mrs. Hudson says he has a brother somewhere
Who should be notified soon
And all the people said; What a shame that he's dead
But wasn't he a most peculiar man?

The suicide of the world's only consulting detective made headline news. People publicly mourned and Lestrade made a tearful statement to the press. He had a lavish funeral as was prepared for by Mycroft. The whole event was covered. There was a second page story about a war doctor who had just come home committing suicide. Shot himself in the temple with his service pistol wearing full dress uniform. The officials said it was his PTSD acting up. No one said a word about his death.
In the official obituaries, however, the name 'Dr. John Hamish Watson' was tucked right next to 'Sherlock Holmes'. In a week no one ever thought of these two peculiar men again.