He burst into room after room, finding them all empty, fear rising in his chest. Something bad was going on, he just knew it. He was pretty sure the serial killer didn't bring Sherlock here just to talk to him, and whatever bad things Sherlock had done in his life he didn't deserve to die, no one did.
He crashed through yet another classroom door, this time catching a glimpse of something out of the window that made his heart turn cold. He was in the wrong bloody building, he should have took the other one, because there was Sherlock and the cab driver standing near the window of the room opposite.
John ran to the window trying to see what they were doing, narrowing his eyes, squinting. It didn't look like Sherlock was in immediate danger but then he noticed the tiny pill in his hand, millimetres away from his mouth, and another one in the cabbie's hand.
"Sherlock!" He shouted through the windows but of course he couldn't hear. What were they doing? He knew the other victims had all been apparent suicides but Sherlock had discovered them to be murders so was this how he did it? They took pills together and one of them ended up dead? Whatever it was John knew there was no way he could let Sherlock take that pill.
Before he'd even had time to think it over, the gun had been whipped from inside his jacket pocket and John was aiming right at the cabbie and he fired once, straight into his chest. As soon as it was done he ran from the window, darting down the stairs to make his way into the other building and find Sherlock.