This is big. Was anyone on the phone with 911 saying "OK, looks like someone took out the shooter and is holding them at gunpoint" and did that information fail to get to the officer?
Very tough situation in general as an officer bursting onto the scene. I think this one will result in no charges being filed, neighboring police department will investigate and say that cop saw a person at a reported shooting pointing a gun at another and acted to save a life (to his perspective and with the facts he currently had to base his decision off of)
Very sad for the hero security guard who lost his life. Police departments need to A) be better trained across the board and B) hold themselves more accountable and quickly.
If you want to say the cop was not criminal in reacting wrongly to a scene when under duress and saw a potential murder about to happen, fine, that's an acceptable view (and the one I belive will come about) but this officer clearly demonstrated he does not have the decision making ability to be a cop and so should be fired and told to find a line of work elsewhere.