All I can figure is that when the victim got the cop's Taser, the cop must have been focusing on that fact as he drew his weapon. (I would be thinking the Taser could be used on me to disable me, and the fact the other guy had it would be cause for me to draw my gun as a means an allowable level of force escalation.) (Edit: accounts say the cop had used the Taser already on the subject, so I don't think it could have been deployed again, against the cop, but I'm not sure in the heat of the moment if that entered the cop's mind); It seems strange that the guy who grabbed the Taser would immediately run off with it, though -- and that's, I think, what led to the shooting. The cop must still have been processing the fact that the "bad guy" had his Taser, and focused on that instead of the fact that the guy was now running away instead of turning the Taser on him.
The cop made a huge mistake by firing on a fleeing "felon" (if that's what he was by then, depending on what had happened during the previous struggle.) I think the cop was on auto-pilot from the time he drew until he finished firing. (Eight rounds sounds bad, but they were quick rounds, and I don't fault that part of the cop's actions, because yes, you use deadly force until the subject is down and no longer a threat.)
The cop was completely wrong in the situation. I don't know if a murder charge is warranted, but at least manslaughter. It was a tragic incident, and regardless of other incidents that have happened recently, shows how a single bad day or lapse of concentration or judgment by an LEO can have terrible consequences. I'm not defending the cop; I'm just saying that I appreciate what LEO's do, and can understand human shortcomings that, in a situation like that, are tragic for all concerned.