Lesson #1: Don't physically assault strangers.
Lesson #2: Guns are great for protection.
Imagine the same scenario but you're alone at the ATM and two men approach you accusing you of something you didn't do. They begin assaulting you by throwing punches. Rather than risking brain damage or death (which can happen in seconds), you casually pull out your gun and 'unalive' them. Problem solved.