As much time as I spend running overly aggressive cops through the ringer, for this, I have to say 100% justified. I don't give a damn who you are, if someone has you in that kind of position, beating you about the face, you would have to be a fool to not fire, while you still had a chance.
I see so many factors here that contributed to this turning into what it did, and it really just proves something that all of us here already know. All it takes is a moment of distraction. The situation was clearly never under control, but in that moment that he yelled to the onlookers, he was distracted, lost any advantage that he had, and was taken by surprise, ending up on his back.
Also, at any given point, that cop probably could have hit him with a taser, or enough pepper spray to blister the skin, and wouldn't have faced any repercussions for it. The fact that he didn't, tells me that he was an officer not wanting to use excessive force. It's just sad that this time, not using excessive force, ended with the need for using deadly force.