A police officer shooting somebody that is physically attacking them with a metal club in a violent riot that involves firebombs being thrown at police officers? I would neither be surprised nor worried if that happened in the US.
Now, if they just opened fire at unarmed, peaceful protestors, I'd be worried (though not surprised that much).
In any case, you mentioned it happening in the US, that's what I was commenting on. I cannot imagine US cops not using their guns when attacked in that way. There is a point to be made about general strategies and not letting it escalate to the point where protesters throw molotov cocktails on police, but I don't know that either UK or US cops would simply disengage and give up parts of town to mob rule.