Yea, being vet I'm sure played a part on him being found guilty. He has training that most of the public does not ever get.
But let's say for argument sake, he was "protecting" the other employees as you say. He wasn't real concerned after he left a suspect in his store alone with the other employees when he chased down the other guy. That threat was gone. He should have stayed to make sure he didn't get back in. Or that the guy laying on the floor didn't get up.
He wasn't concerned when he walks past the kid on the floor with his back turned to him. Walks to counter, reloads, walks back with his guard down, and then executes a non-threat.
Protecting the other employees was never on his mind. He had one thing on his mind, revenge. And now, because of his uncontrolable anger and revenge mind set, he is going away for the rest of his life.
And I think your forgetting the main thing. He shot and killed the kid that never displayed a weapon. Not saying the first shoot wasn't justified, but after the actual threat was gone, again, he went too far.