According to legal analysts and law enforcement commentators on the various MSM cable news channels I've been surfing, an officer is authorized, under certain circumstances, to use deadly force in the course of apprehending a felon or a suspected felon, even if the felon or suspected felon is fleeing.
I need more information at this point, though, in order to decide for myself if the use of lethal force was justified in this case.
It's been reported that the officer sustained an injury to his face during the initial altercation. If true, it's entirely possible that MB attacked the officer while the officer was in his patrol vehicle. Attacking a police officer is obviously a crime, although there are degrees ranging from 1st degree to 3rd degree. See below link for info regarding assault of a law enforcement officer.
http://www.moga.mo.gov/statutes/C565.HTM
There have also been reports that the officer claimed MB was trying to take the officer's gun during the initial altercation. Even if true, MB obviously was unsuccessful in taking the officer's gun, since MB was shot & killed with the officer's gun.
http://www.theguardian.com/world/2014/aug/15/michael-brown-when-can-officers-shoot-ferguson
At this point (based on the currently available information), I'm not persuaded that the circumstances meet the criteria for the lawful use of lethal force outlined in #1 and #2 in the above article.