I understand how it looks...it's easy to think that he did everything wrong. Everyone is real comfortable with the cop reacting too slowly to a threat and getting himself shot....except for the actual cop.
I don't know who mentioned it earlier, but was this security guard holding the subject down and had a gun pointed at the subject?
That's the most confusing thing about it to me. I can understand why he would use the gun to subdue the subject....but once he's subdued, there's really no reason to keep your gun out and pointed at him. Unless he literally just got control of the guy before he was shot by the cop...you'd think he would holster his gun and use both hands to keep control.