Apart from his drugged state impairing any sort of immediate reaction, I imagine that once he came to his senses, Geary would, quite simply, have shit himself in terror. Killing the hooker wasn't only done to blackmail Geary, it was to demonstrate to him just who he was dealing with & what they were capable of.
So dead hooker=horse's head. I never thought of it that way, but it's an excellent point. I always thought of it as simple blackmail.
