Even if the black man was in very good shape, for a 50 year old, and had gotten away, he would have been inevitably picked up on something else and they could have added the running away charge at that point.
here’s the problem with just letting him go. he’s running toward a pawnshop and other retail businesses. he’s run from a traffic stop, ignored commands to halt, resisted arrest, committed several violent assaults on a police officer, stolen a weapon (at that point Slager believed Scott had his taser)
Scott is getting tired and he’s not running very fast. his decision making is getting reckless and he’s committing increasingly serious crimes.
now he’s headed toward a retail parking lot with people getting in and out of cars. he has a taser so that’s a risk for carjacking. there’s also the pawn shop which probably has lots of guns
letting him go means exposing the public in that retail area to the risk of serious injury or death - which is the Supreme Court standard for use of deadly force.