My thinking on the rules of engagement is that if two people begin shooting at each out, somebody's got to die. Either that or one party runs away or throws the gun away to surrender. There is no giving up with the gun still in your hand. At that point you are still a very serious threat.

I also think that you are underestimating how fast these incidents happen and severely overestimating the speed of human decision making. And thus you are expecting way too much out of the cop.

I'd be willing to bet that all these shots occured in a span of no more than 10 seconds. In that time span, you expect the cop (who is being shot at and in the heat of the moment), to recogize that the guy who was just shooting at him is no longer a threat to him.