If i thought my life (or anyone else's) was in danger from a dog, or any other animal, i wouldn't hesitate to do whatever i could with whatever i had access to. I think, even with no witnesses, you would be ok as far as the law was concerned, unless the dog/animal owner could prove your story false.
As for the cat, again, unless foul play could be proven, i don't think there would be anything the cat owner could do. But, i would hate to think you were planning something like that poleking?!.
I was at a customers the other day, an old lady, who had just had her garden done up. She was pointing out these battery powered cat deterrent's she had in 4 places around her garden, said they work a treat, no problem with cats, and the battery lasts most of the year.