He inserted himself into a verbal situation, and promptly escalated it by introducing force and violence.
He quickly rose to his level of incompetence, and found himself physically outmatched,
so he escalated the violence further to the point of deadly force, resulting in an unnecessary and unjustified death.
We'll, there seems to be a lot of pent-up vitriol there, so I'll may bow out of this subject after this post, because if a person thinks that cops are generally bad then there is no where to go with the discussion.
But a couple of points, just for discussion. First, it was not a verbal situation. The cab driver had already been punched, so blows had already been exchanged. Once a police officer becomes aware of someone obviously very intoxicated in public and that has already come to blows with anther person they are expected to engage. This scenario of a single officer arresting a drunk person happens all the time and is not generally out of the ordinary.
It is a tragedy, and it did get out of hand, but it is also very easy to second-guess an action in hind-sight. Was there an original intent to escalate to deadly force by the officer? In this case, I can't imagine that there was.