Nowadays people don’t just slap you and you can slap them back and walk away. There most likely to beat you as hard as they can. So if you don’t hurt him badly he might come back with a weapon, what you should do at the circumstances?
You must not exceed in defending yourself. therefore, you can use what is necessary to defend yourself but without going out of your way to be as evil as he is.
A lot of people would take this and consider preemptive attacks as legitimate ways in defending oneself while it is actually legitimizing hostility and attacking others.