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.

Weathering a sex scandal would have been one thing (he was, after all, an actual customer of the brothel), but any further action against the Corleones would have placed both him and his family in mortal danger. Much, much easier to play ball and reap the benefits.