As for causal culpability, the dog owner is clearly (imo) the heavy.... but I also lay a lot of the blame on the porky cop on the sidewalk. He let the dog owners words completely blind him with what I call 'non-compliance rage'. I'm sure he told the dog owner to stfu more than once, but you can see in the exchange that he first starts storming down the sidewalk, but stops. why? who knows - maybe his partner, the non-porky cop, tried to talk some professionalism into him.
Well, that only lasts until the next verbal barrage by the dog owner and you can see he comes charging anew (soon followed quickly by his partner). Watch the porky cops eyes - he is pissed and is intent on showing the dog owner who's boss.
Note that he sees the dog-owner putting the dog into the car before the confrontation, yet porky almost completely ignores the car/dog as he is 'teaching the dude what non-compliance to his orders gets him". The dog starts barking as soon as they cuff the dude - porky glances at it after it barks. He shouldve easily seen that the window could not contain the dog if the dog really wanted out...
He THEN charges over to the dude with his AR-15 and cross checks him backwards after he is already cuffed.
THAT is what got the dog to come out of the car. After that it is already too late.
imo, porky - not the cop who shot the dog - shoulders most all of the LE portion of the blame pie. He was virtually unaware of what appears to be an obvious personal/public safety concern. (i.e. snarling big ass dog/halfway open window)
He also was the one who took it from some dude yelling insults at the cops, to a dog being dead. He grows a thicker skin, with a dash of understanding of why the dude is yelling insults, and nothing dies that night.