Believe me, watching these things make me sick.  Just utterly sick.  At first, I didn't know of what went on after the shooting part (the kicking, foot on the shoulder).  Also, whether one believes the decent cops or not, it is a common refrain of decent cops to yell, "STOP DOING STUPID SHIT" when learning of the idiocy of these acts. 

It is going to be interesting to watch how this one goes forward.  I just read last night where supposedly the same DA commented some time ago that when a person gets ahold of an officer's taser, then that is considered a deadly force issue, but I do not know the full story of any such comment. And I would think any such comment would have came with certain caveats (like an officer knocked to the ground, alone or some other type of incident that left deadly force to be the last resort). 

I will suggest this - I think with the particulars of this case it would be hard to show a deadly force issue because there are two officers involved, both were not incapacitated and the suspect was fleeing.  My point is, I can see a deadly force issue being raised when there is just one officer and a suspect obtains a taser, or if say one officer has become incapacitated and the suspect is about to commit some further act of violence on the incapacitated officer and the other officer has no other choice but to use deadly force.  But to use deadly force in this specific incident - man, just let the guy get away to be caught another day.

As for your main question - that is a tough one, but it's almost hard not to wonder if a lot of the actions were not related to some sort of felt attack on one's ego.  The kick, the "I got him" and shoulder issue seems to imply that. 

What is so baffling to me anymore is that we all know that cameras and the capability to record things are EVERYWHERE and the more troubling part to this is that even that doesn't seem to deter bad behavior.