I think there would also be a common instinct to protect. This is basic decision made at gut level and probably would not require careful consideration. IMO, for some reason, at that time he did not think the co-workers needed protection. That would work against self defense at that moment; and perhaps that is why he did not kill him then.
Again, don't know what the dude was doing out of sight after the shopkeep left.
In the movies;after those hero vs villain hand to hand fights, they always have the part where the hero thinks he has killed the villain and starts to walk away. But we all know the villain is going to roll over and grab their ankle one more time. I am always yelling stab him again, don't be a fool he is not dead, you have to kill him!
Perhaps this was just such a situation, where the shopkeep thought he had taken care of the intruder and then realized he could still hurt someone or something.
I don't know, but suspect we will find out.
I think that cliche ending is one of the worst of the myths we tell ourselves, J. But we'd be hard pressed to find an action film over the last 30 years that doesn't end with such a killing. No wonder support for the d.p. is so high here.