Possible. But do you think the officer had more than one round in his pump action shotgun? Do you suppose a blast from a shotgun fired in close proximity as a warning shot would stop the pooch in his tracks? A shotgun is pretty damn loud and unless the dog is trained around them, it will cower at the sound.
Hellfire, even the most vile, evil, child raping, murderer fleeing from police gets a warning shot. And I consider this dog to be a higher form of life than that.
The fact is that this cop is a lowlife and made a snap decision to fire before assessing the situation rationally. Take his guns away and put him behind a desk. His next shooting may involve a kid with a candy bar that he supposes to be a weapon.
That's all the time he had. Most decisions to fire have to be made "in a snap."
If you think that a police officer should hesitate to defend himself from serious injury rather than kill a dog, then you might as well join PETA.