It's one thing to call the police and ask the man to stop hitting the woman and to leave your store. Those would be appropriate actions.
Slapping a woman in the face is not an appropriate behavior but it is not life threatening. If the man had assaulted(or attempted to) the women in a way that could have resulted in serious injury or death then intervening in the manner this man did would certainly be justified. In the case of a simple assault one can afford to wait for the police to arrive.