A sober reflection of what was happening would have led to the conclusion that that was not the case -- however you don't have the luxury of Monday morning quarterbacking in the heat of the moment. Surely a monumental mistake, but not the execution that was first proclaimed (and which I bought into when I saw the first photos which did not show the taser wire sticking out of Slager's leg).
Perhaps the first shot or two can be explained that way. What about the remaining shots as Scott is fleeing? At that point if Slager thought he was in danger of being tased, he could have spent his time removing the barb from his leg and/or chest.