People get sued or go to jail because they go too far in "self defense." It is the extra kick when they are already on the ground that will get you in trouble. Or the pre-emptive punch becasue "he looked like he waws going to hit me." These are the bar room-type fights that can often be avoided anyway by just backing down. As for a mugger, tho, let's be realistic. How many people are mugged by someone without a weapon? If the mugger has a weapon, virtually anything you do to defend yourself will be ok, and if the weapon is a gun or knife, virtually no martial art is proof from getting yourself killed.