"I couldn't bear the idea of you being dead. For three years I thought everyday would be my last. So many times I stared at my gun, the knife, the rope, the roof. But then I realized how stupid it would be for me to take my own life. So stupid because I knew you were alive. Out there. Somewhere.
"But you still never came.
"So I decided to look for a way to lure you back here. What better way than this?"
John Watson opened his arms, showing the bodies at his feet. Sherlock noticed the mad look in his eyes, as if he'd lost his head in his absence.
John saw Sherlock watching him so intently, so concerned for his friend, and smiled.
"Welcome back, Sherlock Holmes."
Sherlock was speechless, his eyes flying between the bodies on the ground and the look in John's eyes. He remembered the anonymous tip he had received about the killer. He'd thought he'd recognized the handwriting, but hadn't thought it was all that important. Now, looking back at his mistake, he cursed himself for not being faster. Smarter.
"Surprised?" John said, too loudly in the silence of the abandoned subway. Sherlock winced, coming back to reality.
"No..." Sherlock said softly, knees shaking. He knew his friend could kill. He was a soldier in Afghanistan, for god's sake! But he never knew his friend would fall so far without him there to stop it.
Sherlock got a quick flash back to the first time John had killed for him, on their first ever case.
This was the same... but different. This time, instead of killing out of pure fear and defense, John had been killing out of grief, and, in a twisted way, hope.
"Well... I solved the case. Your case. What happens now?" Sherlock said, a little louder this time.
John looked up, slightly startled.
Just as Sherlock thought. John had been so caught up in trying to lure Sherlock back to London (and back to him) that he hadn't thought his plan all the way through. This meant that no matter how confident in his plan he seemed on the surface, he still had enough doubt on Sherlock's arrival that he failed to plan past it.
"Now?..." John said, sounding lost. He looked down at all the bodies at his feet. "What happens... now?"
"Yes, John. Now. What do we do, now."
John looked up and met Sherlock's eyes at the word "we." While he hadn't thought in depth that far into the future, it had occurred to him in passing thoughts that the second Sherlock figured out who the serial killer was, he would have John locked up in a cell somewhere remote and desolate. But now, Sherlock was asking John what he wanted to do next, giving him a choice. Giving both of them a choice.
John faltered, his confidence (and insanity) slipping away with every word. "I... I-"
"Quickly now," Sherlock said, walking up to where John was standing and pulling him by the arm, "I told Lestrade where we would find the killer, and you standing on a platform made of bodies doesn't look very good for your defense."
"Defense?..." John continued, looking even more lost and defeated.
Seeing his friend like this made Sherlock soften a little. "It's alright, John. I'm back now, and it's all going to be okay."
At this, John broke. He sobbed harder than he ever had in his life. Harder than when he had been shot in the shoulder, harder than when Sherlock had jumped. He fell, Sherlock's grip on his arm not strong enough to keep him standing. He cried harder and harder, until he was curled into a ball on the ground, screaming. Sherlock guessed it to be remorse for what he had done, and gave him time to cry. After around 10 minutes of hearing John's cries, and knowing the police couldn't be very far by now, he grabbed John's arm, harder, and yanked him to his feet. John's head snapped back, and after regaining his footing he stared up at Sherlock, looking stunned and dazed.
'He's dissociating,' Sherlock thought, shaking John hard by the shoulders. A little bit of the life came back into his eyes as he continued to stare at Sherlock.
"Stay with me John." Sherlock said, pulling his friend behind him as he started for the grate on the wall of the subway. While it was the first place Lestrade would look after seeing no one was there, it would have to do for now. Leaning John against a wall, Sherlock took the grate off the wall. He then pushed John into the tunnel, and pushed the grate into place behind him.
"Go." He said quietly, hearing the thundering steps of the police entering the subway. For once, Sherlock was glad for all the noise they made.
"What?" John asked, looking enormously confused and terrified. Sherlock deduced that his terror had more to do with Sherlock leaving John alone than actually getting caught, and Sherlock felt a pain in his chest. He rubbed it, hard, willing the ache away before feeling strong enough to speak without his voice cracking.
"GO!" Sherlock said again, whisper-yelling and pointing through the grate to the blackness of the tunnel. "GO NOW!"
And so, Sherlock watched as John half-ran, half-stumbled down the tunnel towards innocence, and with that, freedom.
Sherlock then turned around and gave the police the best explanation he could: "false tip."