I just watched this a few times with a few people, and we all believe the Officer acted appropriately in shooting the dog as the dog was attacking.
Let me give you another perspective.
The dog owner was shouting things at the officers, no doubt in an attempt to do more than just grab their attention. You can hear him saying things like white cops, no black cops and civil rights violation. It appears as though at least once the officers tried to warn him or something along those lines. Owner places his dog in car, then walks towards approaching officers, saying who knows what.
Naturally, the officers detain the man because of his behavior. Placing someone in cuffs does NOT mean arrrest, but rather detaining. They search him, and of course the dog wants to protect his owner, that is instinct. The officer puts his hand out and attempts to grab the leash of the dog, not entice it. Since the officer is a stranger and his owner is in trouble, the dog reacts instinctively and attacks. Officer reacts to preserve his well being and uses the first thing to achieve this which happens to be his gun, and shoots.
Humans and dogs both have similar instincts. Protect yourself and those you love. Both the dog and the officer did just that. Dog was going to bite, officer had a gun. Is it unfortunate that this happened? Absolutely. But I believe the officer had every right to protect himself from an attacking dog. If he doesn't, dog grabs a hold of him and doesn't let go until when? He gets shot by another officer??
Police officers deal with scum every day, and are trained to always be ready for the unexpected. They were right to detain (again, not arrested - that does not happen until miranda is given) the dog owner and act to protect themselves.
Yes, I have training in law enforcement. I also get to see the bullshit scenes officers have to deal with and the potential for a calm situation to turn into a cluster as I work alongside them often.
Of course the owner will sue, and of course he will win. We live in a ridiculously litigious crazy society. People sue because their fucking coffee is too hot and they burn themselves and win.
It is hard to see it the officers way unless you've been there or experienced something similar.