As soon as Sherlock saw the slight inclination of John's head he knew what he was going to do. Unfortunately, it had likely filtered through Moriarty's mind that Sherlock had only the one option, and the former was rather a lot closer to the nearest exit. Not that Sherlock cared overly; at least he wasn't bored. Oh no; Moriarty had seen to that. It was as though the man knew what it felt like to be dead with the dullness of the world, as though he knew that the mind was a fragile thing, so easily snapped by the heavy grip of the boredom that had so often stolen Sherlock's health. If Sherlock's mind snapped he didn't know what he would do; his mind was all he had, that twisted labyrinth of never-ending thoughts and theories and stratagems, the entwines of wonder and intellect and knowledge that made him all that he was.
But even then, this wasn't about that. Not really. Sherlock had felt something strange stirring in his stomach since the other man had walked back to the pool side and the little red light had passed over his chest once again. Because at that moment, Sherlock had been taken utterly by surprise. That was not something that happened to him, and Sherlock hated Moriarty for it. Nobody was cleverer than him. Nobody wrong-footed him. He was Sherlock Holmes, the greatest mind in the world. He could work out anything, could see anything, could predict anything. But he hadn't even considered that Moriarty would walk back in. Why would he? Without Sherlock, what was he? Clever. He was just clever, and being clever wasn't enough unless somebody else was clever too.
Sherlock had not wanted to kill Moriarty; he didn't want to lose possibly the only mind on the planet capable of rivalling his (he refused to count Mycroft, because his brother was different, his mind was not like Sherlock's, it was not like Moriarty's). He didn't want to be bored again. But the flash of red, the breath of John in the chlorinated air, the grating laugh and the hideous knowing look in Moriarty's eyes was almost more than Sherlock could bear.
His hand had lowered, the gun pointing steadily at the jacket. Dark eyes dropped to stare at what he was about to do, his mind suddenly still. He could sense, rather than see, John's eyes on him, wondering if the detective was truly reckless enough to shoot. He could feel the pressure of the air on his skin, slightly too warm to be comfortable, the smell of the chemicals familiar and comforting. He could hear the soft breath of the sniper behind and to his right, calm and ready. But worst of all he could already feel it creeping in; the boredom. The blackness that was capable of sucking him under, the feeling that poisoned his mind and dulled his senses. The feeling that made life not worth living. Boredom. Sherlock wouldn't allow it to happen.
He wasn't even sure of where he was anymore; all that existed was him and his mind and the poison that was even now beginning to stain the edges of everything he knew. Boredom. Somehow, if he destroyed the jacket, he knew he could destroy the boredom. If his finger pressed, it would all be gone; that jacket was his boredom, and to shoot it would be to make him utterly invincible. He could make his mind truly his own, would never have to surrender his thoughts again. He would never feel so cold inside, so lost and so bored.
The movement ahead of him didn't register, because Sherlock's will was no longer his own; he would never be bored again and he would pull the trigger. There was a bang that echoed inside his head, an impact and another bang. Something hit him and Sherlock became aware, suddenly, that there was fire in his blood. Worse still, there was fire in his mind; it was all he could see, all he could feel and he wasn't even sure if he was alive anymore. His thoughts were burning, twisting and dying as embers before disappearing into ash that floated away into a blackness that Sherlock followed into, giving himself over to the fire because at least he wasn't bored. He would never be bored again.