It is understood, and sometimes inevitable, that harm may be done collaterally in any act of force. If, say in your flash-mob situation, the interveners aim at suppressing the looters by effective limited force, and end up in a chaotic scene in which looters and onlookers are injured and killed as well, that is bitterly sad and heart-breaking, but it is not murder -- if the cops did their due diligence of trying, however unsuccessfully, to employ only a limited and rational use of force.
However, if they just decided to wipe out the problematic neighborhood itself with incendiary devices which took out looters, onlookers, shopkeepers, private security guards, news crews, passers-by, and neighbors for 10 city blocks, you can bet that whoever made the decision would be sacked, and possibly criminally prosecuted.
(BTW, Collateral harms, including deaths, can come about from the decision not to act with force --- in other words, by omissions --- as well. If the cops stand around and watch looters set fire to shops and assault business-owners with paving stones, and fail to intervene with force, they share the morally culpability for the losses, injuries and deaths.)
In the OT, it is repeated over 20 times that God hates -- hates--- the shedding of innocent blood. The same word --- "abomination" --- is used to describe the shedding of innocent blood, as is used to describe blasphemy, idolatry and sodomy.
This is a matter of Divine Law and also of Natural Law: what people can know as rational creatures. A philosopher I mentioned in another part of this thread, G.E.M. Anscombe, found a surprising degree of consensus on this going back to Aristotle. Surveying the whole history of ethics, both Christian and pagan, Anscombe says:
"There is one consideration here which has something like the position of absolute zero or the velocity of light in current physics. It cannot possibly be an exercise of civic authority deliberately to kill or mutilate innocent subjects.'
"For men to choose to kill the innocent as a means to their ends is always murder."
This is a consensus statement of what "murder" is. If intentional or deliberately indiscriminate killing of the innocent isn't murder, then nothing is.
This is completely false.
It is not the position of law enFORCEment to try, judge and sentence whom shall be arrested and taken into custody, either by deadly force or coercion.
The executive is to exercise force to preserve law and order. Frequently the best defense is a good offense in modern firepower and small unit tactics. No LEO would be guilty of murder if placed in a situation where criminal lawlessness is underway and use of deadly force is authorized and his duty is to enforce the law. As part of his rules of engagement he might issue verbal orders to control the public behavior as an arrest, but if not obeyed, he would be authorized to use deadly force and not be encumbered to judge if maybe the criminal really might be a nice guy if given more grace.
A convenient check on this thinking is to consider how you would expect the innocent to respond to your actions if you invaded another person's home, or store, with friends in a coordinated assault, and began looting. My natural expectation would be for somebody to not only stop me, but possibly shoot me for even being there.