So they are trained, but in this case there wasn't necessary to shot a man (in the back). He could shoot on the leg, and conclude the story with a simple arrest but all we know that the us police are thinking "I'm a hero".
Unfortunate incident. The cop had decades of experience in the police force with him. But a few seconds of misjudgment has resulted in the loss of one life.
They're still human beings their decisions can be still flawed by emotions and egos thus they can still make stupid things.