Watson acted quickly; he was a man made for action, after all. When Holmes was in peril, too, he didn't have to think. There was only one thing he could possibly have done, after all.

His hand was already on his revolver, and in a heartbeat he had rushed out with it at the read. Even in the heat of the moment, even after witnessing the man assassinate his double across the road, he couldn't quite bring himself to shoot the man in cold blood. There was no way to do such a thing safely, either, not with him struggling with Holmes. He would sooner cut off his own hand than risk injuring Holmes.

So, before he could have consciously thought any of this, Watson rushed up behind Holmes and Moran, struggling on the ground, and brought the butt of his revolver down upon Moran's head, hard.