I'm going to confront this dog's owner
What did the owner do for you to confront that person?
The dog attacked a cat and bit a boy
It is good to call out the bad behavior of the dog. It sounds like the dog could potentially hurt people so it needs to be trained better.
I'm going to let the dog off the leash to let it attack the owner.
I don't think that's wise, if you did that the dog could cause a lot of harm! I think you should just talk to the owner about the dog's behavior.