I figure his first shots were off because in the excitement, he may have been jerking the trigger instead of drawing it back slowly. I’m sure he was shaking as well. I know I would have been.
Regardless of why, fact is he DID shoot Brown four times in the arm - and that did NOT stop Brown. That, for all practical purposes, addresses the “he shoulda shot him in the arm/leg before resorting to more lethal shot placements” - he did, four times, and was STILL in immediate threat of _further_ grave bodily harm (having already suffered a skull fracture).