McGlockton was stepping back when shot. This is my only problem with the shooting. I can clearly see he's stepping back and Drejka would have seen that as well. There is ample time from when Drejka pulls his sidearm, to the time he shoots. McGlockton was stepping back and away during that time, indicating he was either waiting for Drejka to stand up to continue fighting him or he was disengaging completely. We'll never know for sure.
Mr. Drejka, apparently makes it his mission to berate people when they park in the handicapped spot there all the time. He wouldn't have been in the position to be shoved in the first place if he had minded his own business. This was completely senseless on both of their part. McGlockton could have told Drejka to kindly go fuck himself and to leave his wife and child alone.