My opinion from experience. (May contradict case law / code)
http://www.kmbc.com/r/24836888/detail.html
After he stopped the threat he should not have shot at them.
1. I disagree that the threat was stopped just because they were leaving. As long as they are in the same area as you they are a threat. I don't believe it is possible to point to a single action (such as turning around to leave) and claim that is the point where lethal force must stop.
2. If you've ever been in a shooting incident, you would know the brain begins processing information in a
completely different way. (often called flight/fight). When someone enters this 'zone' it is unreasonable to expect them to register the importance of certain actions. Information is pre-processed by a portion of the brain that bypasses the normal judgement center of the brain.(In this case, after the clerk started shooting at the BGs. Even though the clerk visually saw the BGs turn and leave, the significance may not have registered.)
Behavior in the fight/flight mode can only be modified by habitual and repetitive training, such as that Police go through. Those who are not officers can not be held to the same standards.
Because the BGs actions FORCED the clerk into the fight/flight mode, the BGs are RESPONSIBLE for, the actions of the clerk with reduced cognitive abilities.