An officer may not shoot someone in the back five times running away. That cop was at best deranged.
In Washington State the law reads that a cop (or anyone) can shoot a person fleeing from a felony (so assualt, burglary, stolen vehicle, etc.) The idea of a threat to anyone doesn’t have to be there.
However, on other threads there has been discussions of Supreme Court rulings that in some cases will over rule those laws if it is deemed that a person’s rights have been violated.
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.