Without authority, the use of force is criminal.
You're mistaken. Arkansas cops recently opened up on a guy who was driving away from them...yet none of them has been charged with a crime
First, in general, I am not mistaken. If the district attorney is complicit and refuses to prosecute, that only means it is a miscarriage of justice, not that it is any less inherently criminal.
Second, in this particular case the guy was in fact actively attempting to kill people (according to the news article, at least): "Mounted cameras from three police vehicles, led by Officer Vance Plumhoff, show Rickard weaving in and out of traffic, then ramming a police car head-on. The Honda is later surrounded and as the suspect tries to back up, he sideswipes another cop car, and almost strikes one of the officers." Before he rammed the police car, shooting was not justified. After, it was (as long as the guy continued to drive recklessly... if he slowed down the chase, e.g. OJ-style, shooting would stop being justified).