Part 1: Just Turn Around And Go

Sherlock was more than shocked when the person holding the gun on Magnussen turned to face him. He hadn't been this off-kilter, feeling as if the entire universe had turned upside down and sideways, since the moment John had stepped into the pool when Sherlock had been expecting Moriarty.

And here John was again, making his heart constrict painfully, proving yet again that for all his deductive genius, Sherlock Holmes was nothing more than a human being like anyone else: that he, too, made mistakes.

"John," he said, staring at his best friend through disbelieving eyes. No, not John...

"Sherlock," the other man acknowledged, still holding his gun to Magnussen's head. "I was really hoping it wouldn't come to this. I don't suppose you'd be willing to just…turn around, walk out, pretend you never saw me here?"

Sherlock studied his friend carefully. John wore a black stocking-cap on his head; black leather gloves concealed his hands, and he wore a black military-style vest over black shirt and trousers. The perfect outfit for carrying out an assassination. "Now I understand why you couldn't come with me tonight," he murmured with a small shake of his head. Trying to lighten the mood just a touch, he added, "What's with the special services get-up? I thought you were a doctor."

"I had bad days," John reminded him, a quirk of the lips showing he appreciated the other man's attempt at humor. His expression hardened, however, as Magnussen began lowering his hands from his head while John's attention was (apparently) distracted. "Don't," he said in clipped tones; Magnussen hurriedly replaced his hands on his head. John swung the gun around to Sherlock. "And you. Please, Sherlock. Do what I said - turn around, leave, forget you saw me."

"I can't do that, John," Sherlock replied, making sure to maintain eye contact as he tried to simultaneously talk John down and figure out what had driven him to this extreme. "You know that. Even if you tell me why you're doing this."

The only logical explanation was that John was desperate to save someone from Magnussen, and Sherlock had a good idea as to who that someone might be. What he needed to know was what secret the blackmailer was holding over Mary's head that was worth his cold-blooded execution by her husband - only then could he decide whether or not to take John's advice and just walk away. Not that John hadn't killed since being invalided out of the army, but that had been under immediate threat, or at least what he'd believed to be immediate threat.

Although others might argue that John's shooting of Jeff Hope was just as much an execution as this would be, Sherlock knew it was fundamentally different. He wouldn't - couldn't - just let it go. "Whatever he's got on…whoever it is you're protecting," he amended at the last moment, on the off-chance that he was wrong, "let me help." He shifted the weight on one foot, preparing to step forward.

John's face tightened with some unreadable mixture of emotions - anger, regret, resolve? "Sherlock," he warned, his voice quiet - and deadly. "If you take one more step, I swear I will kill you."

Sherlock smiled sadly. "No you won't, John. You're my best friend, remember?"

As he started to lift his foot off the floor, he had exactly a fragment of a second to recognize what a terrible mistake he'd made - and then John pulled the trigger.

Sherlock stared down at his body in shock as a hole appeared in the lower quadrant of his chest, then back at John as blood began pouring from the wound.

"Oh Sherlock," the other man said, sounding choked up and regretful. "I'm, God, I'm so sorry. Truly I am."

"John?" Sherlock croaked disbelievingly, as his friend turned the gun to Magnussen. Then darkness overtook him, and he saw no more outside the realm of his mind palace. The next voice he heard was that of his internal Molly Hooper, and it was due to her efforts that he survived long enough for the ambulance to arrive.

The ambulance that had been summoned by John Watson before he left the same way he'd come - after hesitating, then with a curse striking Magnussen unconscious with the butt of his pistol.

Killing that piece of human garbage would have to wait.