Funny, I was just on my way to add this to the superhero thread.
It serves to reinforce the point I was trying to make in that thread; that there is a difference between instinctive aggression in response to a threat, and losing all sanity resulting in aggressive actions.
She slaps the cashier, with the perception that the counter will protect her from retaliation. You can see her retreat when he attempts to strike back. He probably shouldn't have swung back, but I can't blame him. Him missing, and the physical layout of the restaurant emboldens her to take it to the next level.
Her first step behind the counter was when this went from crazy to insane. She was crossing a line (literal and figurative), with conscious intent to do harm. There is no more "heat of the moment" excuse here. He retreats to a defensive position, until he reaches a point where he feels he has to take a stand, and strikes her with whatever happens to be available. Justified.
It is only at the point where he continues to strike after the threat has been neutralized that he has crossed the line. If his colleague had grabbed/tackled him at the point where he steps in front of the Coke machine, he probably would not have been arrested.