"There's nothing quite like it, though. To be the one on the other side of the weapon. You picked a good one for your first, didn't you? One shot, wasn't it? Right through the brain."
You hold up your hand in mimicry of a gun, pointing your index and middle finger at your forehead and making a whooshing noise with your mouth, like a child might do. A little childish, perhaps, but you know what it must have taken for Sherlock to cross that line, so your imagination writhes with the possibilities.
"Six," Sherlock says, enunciating the word clearly, as if talking to a slow child. "You murdered six people, didn't you?"
"And you've only killed one. Bad luck, Sherlock. But you have time to try and beat my high score. Let's hope big brother Mycroft can come up with some more tidy excuses to keep you out of prison. Wouldn't want people to start pointing fingers at you again."
"I know you didn't kill Mark Foley," Sherlock says, ploughing straight through your barbed flippancy- he cannot allow himself to be ensnared. "The circumstances of death were easy to pin on you, at the time- you left his block of flats that night, after victim three. At the time, you said Foley simply 'got in the way' and so you eliminated the threat pose to you anonymity. Perhaps if Lestrade had assigned your case to me earlier, the details would have come to light much sooner.”