How will gun laws control people who don't obey law?

MARY CHAPMAN raises these points about gun control in an October 11 letter in the Dispatch/Argus."I wish to comment about our president's remarks about controlling guns. ...... First, the man who murdered the college students was not a person enrolled there. Second, the college had no one on campus security who was armed with a gun. So the only person with a gun was the murderer. How is a law going to control a person who doesn't follow the law? ..... In Chicago there were more murders and people injured over last weekend than at that college. When will Chicago (which has very tough gun control laws) stop the murders in that city? ..... How do you control people who refuse to obey the law?