The bad people only need guns because the good people carry them around? How often does having a gun out and about save lives? Not prevent a robbery but save a life? How many times does it escalate a situation?
A sibling comment has already posted a link to Wikipedia.
I couldn't see it mentioned immediately on Wikipedia and it kind of falls outside of the scope of that article too so I'll mention that in addition to people directly preventing crime by pulling a gun IIRC there also seems to be less attempts at crime in areas were there are lots of legal guns.
I think rayiner hints at/mentions that somewhere above.
