Go on. Walk away. Bill doesn't plan on stopping Sherlock. He doesn't have to. The point had never been to kill Sherlock -- which would just drive John away from him. The point is to make sure John stays as emotionally detached from Sherlock as he possibly can.

"Just one more piece of advice," he calls into the darkness as Sherlock leaves. "You really should take a good look at the offices across the street."

There's your final clue, Sherlock.

Likely to do with that marked off passages in that diary that no one will talk about. "Ask John sometime. I'm sure he'd love to give you the tour."