How tangled was the cop in the video? How far away was the victim? Which direction was the victim moving in?
Anyone who would shoot because the situation had gotten awkward, but was no longer a threat does not belong in a job which requires a gun.
The wires on those tasers is something like 25ft
The video shows wires going from the officers torso to the perp, and there is a later photo of the officer with his pant leg up as though he was tending an injury. One theory is that there was a dart in his torso, and/or one in his leg.
If the officer was confused, upset, disoriented, frightened, or whatever, the perp put him in that state by running, and fighting with him. Have you ever been in genuine fear for your life? Time and space do weird things.