Who said he did? You don’t have to wait around to find out if the perp succeeds in incapacitating you or not. You have an imperative duty to prevent it posthaste, genius.
Scott was obviously not an immediate threat re. Garner v. Tennessee, 471 US 1 (1985).
Scott was unarmed, ran too far and was still running. Slager would have been better off trying to follow him until backup arrived. He also had Scott’s car and had a good look at him before the incident.
If there’s no backup (bad dept. politics), the best thing to do is quit and look for work somewhere else. Things will never work out perfectly for all situations. Unusual circumstances are often encountered. There are always options. The more training, the more options.