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.
(Is that what you thought I said? Yikes.)
What I said was that there would be no moral guilt imputed to LEO's who tried to employ a limited, rationally proportioned use of force in a flash-mob looting situation, even if people were collaterally harmed or even killed.
I was making a distinction between inadvertent (accidental, unintended) fatal harm to bystanders, which would not be murder on the LEO'spart, vs. the hypothetical decision to carpet bomb the whole neighborhood indiscriminately, killing all its inhabitants, which would be murder.
I must not have expressed that clearly enough.