If you truely believe that you are in imminent danger of death or great bodily harm you need to do what is necessary to stop that threat.
This case was muddied every way possible. I have a head full of theories about how the plaintiff's attorney could/would try for an excessive force finding if they saw they were going to lose the wrongful death part of the lawsuit.
Remember the apocrophyl story of the guy who emptied the magazine into the armed mugger, reloaded and emptied both spares before he stopped shooting, claiming he "wanted to be darned sure the threat was stopped"? Yes, there is a point when you cross over into excessive force. But apparently the judge did not believe what happened had reached that point - if for no other reason than to prevent the further muddying of what constitutes justified homicide.