Most days Sherlock Holmes is fine. Most days he can occupy his brain with a mystery or equation, and everything is alright. But then there are also days when his mind won't shut up. No equation will suit it. On these days Sherlock is almost goes back to cocaine. He knows it isn't worth it; doesn't make it any less tempting.
On those days Sherlock gets scared. Because those horrible days he thinks about the best way to murder someone. On the worst days he almost carries out his plans. That's why he antagonizes Sally Donovan. Because she is so, so close to the truth; several time they've almost been standing around a body that Sherlock Holmes put there. That's why he pursued Moriarty so doggedly. Moriarty was everything Sherlock could have been.
Now that Moriarty is gone Sherlock gets bored even easier than before.
Moriarty is gone, John is married, and Sherlock starts thinking like he did when he was a teenager.
'It would be easy' 'quick bullet to the head' or 'a simple overdose'.
But, as he told Major Sholto (although no one listened), "we wouldn't do that. Not to John Watson."
So he doesn't do it. He thinks about it though.
I'm telling you this because I've seen his mind, I know how it works, and I understand.
I too know boredom so horrifying it drives you near madness. But Sherlock, well, he's been getting worse and worse lately.
John, I'm telling you this because your gun is missing right now. It's in the hands of a broken man in 221b. So if you're listening, John and I dearly hope you are, hurry.
Because this time Sherlock is his own arch-enemy.