He was back in the war.
For one second, one miserable, never ending, bloody second, he was back in the war.
Heat slams against his body, the impact of the explosion forcing him back a step or two. He catches others moving back beside him, the heat and power causing them to instinctively remove themselves from danger.
For an instant, he can hear the screams, see the blood, taste the sand in his mouth.
A rudimentary bomb, probably constructed by some fanatics in their basement. Triggered by Yarnall opening the door. Yarnall was gone in a flash. So was Halberk. And Reser.
Lambing wasn't as lucky. John remembers the man's screams puncturing the air. Once his ears had stopped ringing, the man's screams were the first thing John locks in on. Soon after, he hears the unit leader bellowing for everyone to move. He sees Beller and Westerfeld move back, but John moves forward. Lambing is still screaming. John has to stop the screaming.
For an instant, he's not entirely sure why he's moving. All he knows is his body is moving and he cannot stop it.
For an instant, Lambing stops screaming, sees John, and thanks God the capable doctor is there.
For an instant, John is baffled. His head is still ringing from the bomb, and he doesn't know how to approach the case in front of him.
Lambing's left side is completely destroyed. Rubble, lying around the two men, had slammed into him, puncturing his exposed skin, ripping it to shreds. His left leg is bent at an abnormal angle. His left arm is splayed weakly out across the stones. Blood is pooling underneath his body.
John drops next to the man, scrambling for his emergency kit. A hand on his own, shaking hands stops him. Their eyes meet.
Kill me.
Lambing is begging him. A feeble cough breaks the sudden silence, followed by the sound of choking, caused by blood flowing into the lungs.
John can't do anything. He can't move, he can't breathe, he can't even understand Lambing's request. He's supposed to heal, not kill. But they both know, there's no healing to be done here.
Hands still shaking, John reaches for his gun, places it against Lambing's temple, and pulls the trigger.
He's back with his unit moments later. They don't speak of it. The whole episode had happened in under a minute. John relives it in a second.
I killed people! You were a doctor. I HAD BAD DAYS!
As John slams back into reality, the exchange between him and Sherlock is at the front of his mind. He's grateful he's behind the detective, that Sherlock can't see him momentarily gasp for air. Besides, if Sherlock had seen it, he would have chalked it up (incorrectly, for once) to the impact of the bomb.
The quartet stares out at the aftermath of the bomb. Henry's relieved because his father's killer is gone. Lestrade's relieved because this whole stupid incident is over and he can return to the more normal cases. Sherlock's relieved because he's right, and he'll never again be troubled by the hallucinogenic drug.
John's relieved because he's in England. He allows himself one, brief moment of relief, closing his eyes and breathing deep. He doesn't know if Sherlock notices it. At this point, he doesn't care. All that matters right now is in England, not Afghanistan.