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Nightmare Fuel: Elementary
The plan from "Red Team". Sufficiently plausible that the government classified it immediately; sufficiently terrifying that a member of the team was prepared to kill everyone else who knew what it was; deduced by Sherlock in a few moments. Of course, Sherlock would never reveal it... but what one smart man can figure out, another smart man can also figure out. The sort of smart man who might, say, casually employ an entire stable of serial killers to further his own ends...
Why shouldn't we worry? As if men have a monopoly on murder.
M aka Sebastian Moran. He ties his victims up, hangs them by their feet, then slashes their throats and lets them bleed to death. Sherlock states that M's youngest victim was twelve.
It actually gets worse after the reveal that Moran is a hitman. Moriarty is directing him to kill people whose deaths are somehow vital to one of Moriarty's criminal schemes, and simply making it look like the work of a serial killer in order to throw off suspicion. What kind of monstrous criminal scheme could require the deaths of children?
Perhaps that particular child was the heir to a fortune.
The Dissonant Serenity of Moran calmly singing the Arsenal FC anthem to himself in his cell before suddenly and violently bashing his face against the mirror.
Not long before, Holmes visits Moran for help with a message he needs to decode from Moriarty. Moran claims he cannot translate. Sherlock translates it later:
Before that, Moran's killing of the guard simply for overhearing bits of his conversation with Holmes. And with such carefree glee afterward!
Can you ever really be sure about which of your loved ones is genuine and which is a carefully constructed fašade who may not be the world's greatest criminal mind in your case, but still wants nothing more than your utter devastation?
Sherlock himself in "M". During his little rant to Watson, he was somewhere in between Tranquil Fury and just plain crazy.