To me the message is clear. You can shoot to kill, but only while defending yourself.
I suppose the other message could be don't lie to LE about it, and be sure to turn off the camera if you do. I don't mean that to be snarky, I really think that is one of the messages to shopkeeps and other pharmacists.
IMO, where this departs from a pure self defense case is that Ersland was collected,deliberate, and unwaivering in his actions. he clearly took control of the situation, evidenced by one robber fleeing and one robber being shot to death.
Even with all that said, I also see the injustice in being locked up for life since he clearly did not initiate this mess. I understand that premeditation involves the time that you can change your mind and continue anyway and that was not difficult to prove. But I still think the fact that he was not the original aggressor, he was threatened with a gun and was protecting others in his establishment should have been enough mitigating factors to rule out 1st degree.