If police officers were not trained to perform such an unnecessary act in the first place, we would not be talking about such hypotheticals.
There may be situations where firing upon a vicious dog would be prudent, but we've been reading more and more of these stories over the last couple of years and the pattern is a very disturbing one.
They are being trained to shoot dogs at the slightest whim.
The question we should be asking is why are they being trained this way.