This is a simple concept, and a fact that is borne out in many statistical studies.

As has been stated here in many previous comments, gun control laws only deter the law-abiding citizen, not the criminal.

Stripping law-abiding citizens of the right to keep and bear arms simply creates more potential victims (rather than potential defenders). The mere fact that a person may be armed is sufficient to deter crime in many cases, and encourages would-be criminals to seek areas having more stringent gun control (where the likelihood of encountering armed resistance) is greatly reduced.