The difficulty becomes developing that justification on the fly. When authorities caught up with the first Boston bomber they attempted to take him into physical custody after a prolonged chase where they were using both explosives and guns. If the police knew they were chasing the Boston Bombers, who they had just engaged in an armed battle, then shooting them while they fled would be legal.
Last year a woman and her son got into an altercation with police in New Mexico. As they fled the traffic stop, one of the police shot at their vehicle. That probably didn't meet that criteria despite some slow speed fisticuffs on the part of a stupid teenager.
So where does that leave this incident? Well there was a fight and supposedly a struggle over a taser. Does that make the suspect a danger to the public? Fighting a police officer definitely rises to another level of criminal, but doesn't mean he will attack other cops or citizens. This fired officer is going to have a tough case. There should be some additional evidence from the taser unit itself. All employments are recorded by the Taser and when the cartridge is fired it shoots out a bunch of little paper discs (chad) so you can tell the approximate location and direction of the shot. The chad also have serial numbers on them.
If, as the man who took the video stated, there was a fight on the ground and the man assaulted the officer and tried to take his weapon, (taser or otherwise), the officer could make the case that the standard of "committed a crime involving the infliction or threatened infliction of serious physical harm, deadly force may be used if necessary to prevent escape" had been met.
If you watch the video carefully, the shooting lasted only 2 1/2 seconds and there was a pause before the last shot and the man was clearly still running before the final shot took him down. Have to keep saying this: not defending at this point, just pointing out the law and the facts we know so far. To say this was simply a traffic stop and the man ran so he was shot for back child support is BS. People should be very careful. I remember a number of freepers defending Trayvon Martin and Mike Brown before all the facts were known.