Saw this some weeks/months back. The owner of the dog was acting like a dipshit, WANTING to be arrested. I don't like asshole cops, and there's no reason a citizen shouldn't be allowed to record video of them, but if you're gonna antagonize police, ya kinda get what's coming.
The dog was trying to protect his dumbass, asshole owner, and unfortunately lunges at one of the officers. An animal like that could do a lot of damage. It sucks that the officer didn't attempt a non-lethal method of taking down the dog, but an officer's gotta protect himself.