Temper
My thanks to Arianedevere for transcribing the unvocalised lines at the swimming pool, arianedevere. livejournal (remove spaces)
The John Watson that Mike Stamford had known had never had a temper. It had been almost an institutional joke at the time, especially since he was also trained as a soldier.
The John Watson Stamford had known no longer knew himself. For he had a temper. A violent one. And it didn't take all that much to set it off any longer.
So far he had yelled at a rather nice old lady offering him tea, had a row with a pin and chip machine walking out of the store in anger rather than attack the thing, partly due to the hefty muscular bloke behind him in the queue, and partly due to the absurdity of it, which even he could see. Although it took him grousing about it in anger all the way back to Baker Street, before he could deliver it to Sherlock, watch his flat-mate's tiny non-smile of mild amusement, and then his sudden generous suggestion that John use his card.
So, when for one of his cases Sherlock asked John to punch him he said the first thing that came to mind, "I always hear punch me in the face when you're talking." But when Sherlock punched John, he hit back, and then attacked the taller man in earnest. And only when Sherlock asked him to stop now, was he able to backpedal and realise that Sherlock actually had meant exactly what he said: Punch me in the face!
But the temper was there, lurking just under the surface. Making him calm when danger knocked and cool when threat was imminent.
It had been his temper that had spurred him to draw his gun when Sherlock was about to eat that damned pill, and it had been his temper that had fired the gun. But it had been cold reasoning calculation that had seen the shot hit as well as it had.
One of the things you learnt when bombs were falling was how to read someone else's lips. Of course there were a limited amount of things people said during a crisis situation, but John had read Sherlock's lips that night as he began to describe to Lestrade the kind of man who would wait to shoot until Sherlock was in imminent danger, a crack shot, a steady hand, with nerves of steel.
And then again at the swimming pool, when Moriarty had returned, and he had been going to pick up the phone, his silent 'Sorry' to Sherlock, who mimed something back that John wasn't positioned to see. ('Oh, it's fine')
But now, his temper had deserted him. There was only a grief he couldn't deal with. A loss that was of more than his self could deal with. Sherlock had become his catalyst even as he had become Sherlock's. John helped Sherlock think better, provided the unexpected angle to Sherlock's mind palace, while Sherlock provided John with the danger he was apparently addicted to.
Being left alone, having been made to watch Sherlock's suicide, there was only one path that John could follow: to attempt to return to the John Watson he had been before Afghanistan. The man Mike Stamford had met and introduced to Sherlock, the man Lestrade knew, and Mrs Hudson and Molly. The affable, kind, generous doctor who never lost his temper. Well, Mrs Hudson knew that he did, from time to time, but she was forgiving of it, because he was 'one of her boys'.
But continuing in Baker Street was not only beyond his means, it was beyond his ability to function if he was to reassemble the man had been before the war, before Sherlock. The innocence was gone, he knew and accepted that, but he hoped that he could re-establish the outward façade as a barrier against the world. Maybe that could get him back with the ladies, he could find himself a wife, make some sort of contribution to the world, and put his wilder years behind him. He knew it was his dotage waiting for him. But he no longer saw any future that was truly worth living, so he had to create one. Or else make Sherlock's sacrifice, whatever it had been for, vain by taking his own life.
He'd try to live first. Then if it truly sucked? He still had a way out.