Two things we know about the criminals. There were three of them (disparity of force) and one made an aggressive move. The first time I watched it I thought he was going after the owner. Why not just grab some merch on the way out? Those two things help the case for self defense. You can always second guess watching someone ten times, but the first time I saw it I did think the jump over the counter was to get to the owner. Now he grabbed cigarettes instead but the owner reacted so fast it was almost simultaneously. I think the problem the owner has is the last two stabs.