It doesn't stop murder and mayhem, but it stops murder and mayhem that involves guns. Guns serve to increase violent incidences because pissed off people can end fights as quick as they need to with them, rather than resorting to (slightly) more sane things like yelling. Would you rather have someone offend you deeply, or end up in the hospital (if you're lucky) as opposed to the morgue? Every country that controls firearms has lower incidences of crime than those that don't. Have you never seen Fahrenheit 9/11?