The larger question is whether or not the officers had any business with the dog's owner in the first place. If the answer to that question is no, then none of their actions have any justification, shooting the dog included.
And unless there's more to the story about what Leon Rosby was doing (if he wasn't simply filming), then the answer to that question is in fact no.
Firing a gun without knowing your backstop is generally bad, but angled down and in self-defense it's not the worst thing in the world.
The dog in question was large and probably could've done some decent damage. It's not right what the officers did, but it wasn't unreasonable either once the animal went nuts.