In the Adventures of the Copper Beeches, the mastiff that Mr. Rucastle warned Violet about did what?

The dog attacked Rucastle.

Holmes and I rushed out and round the angle of the house, with Toller hurrying behind us. There was the huge famished brute, its black muzzle buried in Rucastle’s throat, while he writhed and screamed upon the ground. Running up, I blew its brains out, and it fell over with its keen white teeth still meeting in the great creases of his neck. With much labour we separated them and carried him, living but horribly mangled, into the house.