Had I seen the gun first and decided not to rob that guy, then the fact that I moved on to another victim didn't affect the crime rate at all. I still robbed and killed one guy this month.
Take it a step further though. What happens if all citizens are armed? Then, there are no easy victims. Messes up your cash flow when it's too dangerous to do your "job". How do you make your job easier? Disarm everybody. Then, it's back to the good cashflow.
Penn & Teller nailed it in my opinion...
Most violent crimes are committed by men.
Most violent crimes are committed against women.
Therefore, give every woman in the nation a gun (a pink one specifically). They can keep it, give it away, do whatever... but all of a sudden it's a 50/50 proposition whether or not Janey's got a gun.
...that... and chicks with guns are sexy
