If a dog ran into my house and attacked my cats, I would fend it off any way I had to. I'd kill it if that was the only way to stop it. (I know, I know, they say you shouldn't jump into a dog fight, but . . . I couldn't just stand there and watch my pets get torn apart!) And I would throw every law in the book at the owners. They must be appallingly irresponsible.
StunnedMonkey, maybe it came in through a dog door or cat door? (Cat door would be kind of a squeeze, but you never know.)
Edit: Oh, I just reread and saw that it ran in through the front door.