Great question — answering as the UphillTech team:
I thought the same thing when he fired 6 shotgun slug rounds into the guy rolling down the scaffolding. John seems to be of the belief that finishing someone off always involves at least one in the head or an equivalent finishing tap. But even as far back as the first one, he is known to put more than one in heads if he feels it is necessary. About all we can assume on this subject is that John Wick has a certain reputation that was founded on his exceptional skill in this avenue and because of this we must assume that his choices in tapping are made because he is the best at what he does and during those encounters he was not satisfied until he stopped putting rounds into the victims. So...
Full answer by Kai Qing: https://movies.stackexchange.com/a/101482 (CC BY-SA 4.0).
Need it done for your business? UphillTech.