He shot the assailant several times out of necessity. After all was done and he was able, he immediately sold the gun. He was that dissatisfied with it's performance.
While I get your point, if you don't hit a critical area of the body then likely you will have to shoot multiple times. Most people will do so from adrenaline regardless. For kicks and giggles google Jared Reston and watch the video. Officer shot 7 times with a 45 ACP, once in the face, and still killed his attacker. Why? Hist vest stopped hits to a vital area.