Alone Is All I Have
"Alone is all I have, alone protects me."
He knew he was wrong the moment the words had left his lips. John was right, friends did protect people, but Sherlock's loneliness protected them. Sherlock had every confidence that if John had known what he had been planning – that he intended to face Moriarty, and take his last stand, alone – that he would have come after him, tried to convince him not to go - and if that hadn't worked, come with him. But this really was the only way.
Moriarty would never let either of them live now that Sherlock had discovered his criminal underworld, and Sherlock would certainly never give up his pursuit of the criminal mastermind. He would grow tired of his ridiculous cat and mouse games eventually, and then they would all be in danger – all of Sherlock's friends; John, Lestrade, Mrs Hudson… Molly. The world needed freeing from the trail of evil and poison that the man left in his wake, and so he did the only thing he could think of under the circumstances, he watched John turn and walk away – the man's final words to his friend an infuriated insult "you machine," – but Sherlock knew that he didn't really mean it.
He'd watched John turn and walk away, knowing that it would probably be the last time he ever saw his best friend, but that was alright…
Yes, that was alright, Sherlock reasoned. He'd made his peace with himself, and he wasn't afraid of dying – not anymore.
This may have been Sherlock Holmes' last day of life, but he would never have been able to live with himself anyway if anything had have happened to John.