Whether he needed to shoot and what exactly happened is almost speculation.
That is indeed the question.
According to Fish (quoted) the attack came from about 30 yards away.
Fish should have been able to avoid or trip up someone running at him downhill from that distance.
He had time to yell at Kuenzli about not shooting his dogs, but no verbal warning about shooting him?
Kuenzli had no visible weapon, and the dogs ran off. A situation like this usually means there is a disparity of force. Age, physical condition and size all play a role. Did it exist?
Also, if Kuenzli was not known by Fish, prior to this incident, I can understand why the jury wasn't allowed to hear all of Kuenzli's background.
I don't believe Fish was guilty of 2nd Degree Murder. I believe that he is a victim of either poor training or complacency.
Poor Training-Why didn't he try to seek cover, given a 30 yard advantage?
He could or should have been able to move out of the line of attack and seek cover. If he truly feared for his life, he could have drawn his gun and forcefully notified Kuenzli of the potential results if Kuenzli had still pressed the attack.
The other aspect of his identifiable lack of good training, is his lack of a Self-Defense system or layered response. Pepper Spray of an impact device (hiking staff) could have saved Fish 10 years and Kuenzli his life.
Another training issue is that many schools are guncentric. They incorporate nothing else into the curriculm. No or very limited movement. No avoidance techniques. Just draw and shoot. Draw and shoot. As a result of this, many "trained" people think the gun is the solution to every event where they feel threatened.
Complacency-Fish may have thought his 10mm was adequate for protection from four legged predators, but he was ill prepared for the two legged variety.
This is evident by his statements and actions. Both, by the way can directly relate to his training (lack of?).
Many people become complacent with their firearms. How many buy a gun and don't train or even shoot it to see if it works. If you ask them, they will say they are protected because of the gun, not their training with it.
I am not trying to nit pick this case, but looking at it from an Instructor's point of view, I see things that went wrong. Things that could have been done differently. Things from which we can learn.