There's another aspect we're not fully aware of, that may need to be considered. Watching the video, Mr. Drejka appears to be swaying slightly while he was talking to Britany Jacobs. He may have been confronting her because he needed the handicapped spot, and was entitled to use it, and she did not and was not. If so, as a handicapped person, already unstable on his feet, getting shoved to the ground is a much more traumatic event. That may be why the sheriff declined to arrest him.
Now, that said, I think the sheriff was wrong. Mr. McGlockton, while he should not have shoved Mr. Drejka, may have been surprised when he fell down. Whether he was or not though, he did not follow up on the shove beyond one or two steps, which could have been to simply regain his balance, and was in fact backing up and turning away when Mr. Drejka slowly pulled his firearm and fired.
Mr. Drejka was not in danger of further assault, and was not facing further attack. Mr. McGlockton was de-escalating (even though yes, he had previously escalated the situation). If Mr. McGlockton had continued toward Mr. Drejka after he was down, yes, probaly a righteous shoot. Otherwise, too much too late to justify the shooting.